diff --git a/mod/quiz/tests/behat/validate_manual_mark_correct_format.feature b/mod/quiz/tests/behat/manually_mark_question.feature
similarity index 82%
rename from mod/quiz/tests/behat/validate_manual_mark_correct_format.feature
rename to mod/quiz/tests/behat/manually_mark_question.feature
index 7816bd14af40a..dba965283aa93 100644
--- a/mod/quiz/tests/behat/validate_manual_mark_correct_format.feature
+++ b/mod/quiz/tests/behat/manually_mark_question.feature
@@ -1,7 +1,8 @@
 @mod @mod_quiz
-Feature: In order to mannually mark a question I want
+Feature: Teachers can override the grade for any question
   As a teacher
-  I must be able to set the marks I want on the Rapport question page.
+  In order to correct errors
+  I must be able to override the grades that Moodle gives.
 
   Background:
     Given the following "users" exist:
@@ -18,22 +19,21 @@ Feature: In order to mannually mark a question I want
     And the following "question categories" exist:
       | contextlevel | reference | name           |
       | Course       | C1        | Test questions |
-    Given the following "questions" exist:
+    And the following "questions" exist:
       | questioncategory | qtype       | name  | questiontext    | defaultmark |
       | Test questions   | essay       | TF1   | First question  | 20          |
     And the following "activities" exist:
       | activity   | name   | intro              | course | idnumber | grade |
       | quiz       | Quiz 1 | Quiz 1 description | C1     | quiz1    | 20    |
     And quiz "Quiz 1" contains the following questions:
-      | question | page | requireprevious |
-      | TF1      | 1    | 0               |
+      | question | page |
+      | TF1      | 1    |
     And I log in as "student1"
     And I follow "Course 1"
     And I follow "Quiz 1"
     And I press "Attempt quiz now"
     And I follow "Finish attempt ..."
     And I press "Submit all and finish"
-    # Pop-up confirmation
     And I click on "Submit all and finish" "button" in the "Confirmation" "dialogue"
     And I log out
     And I log in as "teacher1"
@@ -51,9 +51,7 @@ Feature: In order to mannually mark a question I want
     Then I should see "This grade is outside the valid range."
     And I set the field "Mark" to "aa"
     And I press "Save"
-    Then I should see "That is not a valid number."
-    And I set the field "Mark" to "10"
+    And I should see "That is not a valid number."
+    And I set the field "Mark" to "10.0"
     And I press "Save"
-    Then I should see "Changes saved"
-
-
+    And I should see "Changes saved"
diff --git a/question/behaviour/behaviourbase.php b/question/behaviour/behaviourbase.php
index fd7003ed11268..0ddfa5ac3c097 100644
--- a/question/behaviour/behaviourbase.php
+++ b/question/behaviour/behaviourbase.php
@@ -194,7 +194,7 @@ public function get_expected_data() {
 
         $vars = array('comment' => PARAM_RAW, 'commentformat' => PARAM_INT);
         if ($this->qa->get_max_mark()) {
-            $vars['mark'] = question_attempt::PARAM_MARK;
+            $vars['mark'] = PARAM_RAW_TRIMMED;
             $vars['maxmark'] = PARAM_FLOAT;
         }
         return $vars;
@@ -467,15 +467,25 @@ public function process_comment(question_attempt_pending_step $pendingstep) {
         }
 
         if ($pendingstep->has_behaviour_var('mark')) {
-            $fraction = $pendingstep->get_behaviour_var('mark') /
-                            $pendingstep->get_behaviour_var('maxmark');
-            if ($pendingstep->get_behaviour_var('mark') === '') {
+            $mark = question_utils::clean_param_mark($pendingstep->get_behaviour_var('mark'));
+            if ($mark === null) {
+                throw new coding_exception('Inalid number format ' . $pendingstep->get_behaviour_var('mark') .
+                        ' when processing a manual grading action.', 'Question ' . $this->question->id .
+                        ', slot ' . $this->qa->get_slot());
+
+            } else if ($mark === '') {
                 $fraction = null;
-            } else if ($fraction > $this->qa->get_max_fraction() || $fraction < $this->qa->get_min_fraction()) {
-                throw new coding_exception('Score out of range when processing ' .
-                        'a manual grading action.', 'Question ' . $this->question->id .
-                                ', slot ' . $this->qa->get_slot() . ', fraction ' . $fraction);
+
+            } else {
+                $fraction = $pendingstep->get_behaviour_var('mark') /
+                        $pendingstep->get_behaviour_var('maxmark');
+                if ($fraction > $this->qa->get_max_fraction() || $fraction < $this->qa->get_min_fraction()) {
+                    throw new coding_exception('Score out of range when processing ' .
+                            'a manual grading action.', 'Question ' . $this->question->id .
+                            ', slot ' . $this->qa->get_slot() . ', fraction ' . $fraction);
+                }
             }
+
             $pendingstep->set_fraction($fraction);
         }
 
diff --git a/question/behaviour/manualgraded/tests/walkthrough_test.php b/question/behaviour/manualgraded/tests/walkthrough_test.php
index 0e2aec82a323b..95660479a1d40 100644
--- a/question/behaviour/manualgraded/tests/walkthrough_test.php
+++ b/question/behaviour/manualgraded/tests/walkthrough_test.php
@@ -384,4 +384,98 @@ public function test_manual_graded_respects_display_options() {
         $this->displayoptions->manualcomment = question_display_options::VISIBLE;
         $this->check_output_contains('This should only appear if the displya options allow it');
     }
+
+    public function test_manual_graded_invalid_value_throws_exception() {
+        global $PAGE;
+
+        // The current text editor depends on the users profile setting - so it needs a valid user.
+        $this->setAdminUser();
+        // Required to init a text editor.
+        $PAGE->set_url('/');
+
+        // Create an essay question.
+        $essay = test_question_maker::make_an_essay_question();
+        $this->start_attempt_at_question($essay, 'deferredfeedback', 10);
+
+        // Check the right model is being used.
+        $this->assertEquals('manualgraded', $this->quba->get_question_attempt(
+                $this->slot)->get_behaviour_name());
+
+        // Check the initial state.
+        $this->check_current_state(question_state::$todo);
+        $this->check_current_mark(null);
+        $this->check_current_output($this->get_contains_question_text_expectation($essay),
+                $this->get_does_not_contain_feedback_expectation());
+
+        // Simulate some data submitted by the student.
+        $this->process_submission(array('answer' => 'This is my wonderful essay!', 'answerformat' => FORMAT_HTML));
+
+        // Verify.
+        $this->check_current_state(question_state::$complete);
+        $this->check_current_mark(null);
+        $this->check_current_output(
+                new question_contains_tag_with_attribute('textarea', 'name',
+                $this->quba->get_question_attempt($this->slot)->get_qt_field_name('answer')),
+                $this->get_does_not_contain_feedback_expectation());
+
+        // Finish the attempt.
+        $this->quba->finish_all_questions();
+
+        // Verify.
+        $this->check_current_state(question_state::$needsgrading);
+        $this->check_current_mark(null);
+        $this->assertEquals('This is my wonderful essay!',
+                $this->quba->get_response_summary($this->slot));
+
+        // Try to process a an invalid grade.
+        $this->setExpectedException('coding_exception');
+        $this->manual_grade('Comment', 'frog', FORMAT_HTML);
+    }
+
+    public function test_manual_graded_out_of_range_throws_exception() {
+        global $PAGE;
+
+        // The current text editor depends on the users profile setting - so it needs a valid user.
+        $this->setAdminUser();
+        // Required to init a text editor.
+        $PAGE->set_url('/');
+
+        // Create an essay question.
+        $essay = test_question_maker::make_an_essay_question();
+        $this->start_attempt_at_question($essay, 'deferredfeedback', 10);
+
+        // Check the right model is being used.
+        $this->assertEquals('manualgraded', $this->quba->get_question_attempt(
+                $this->slot)->get_behaviour_name());
+
+        // Check the initial state.
+        $this->check_current_state(question_state::$todo);
+        $this->check_current_mark(null);
+        $this->check_current_output($this->get_contains_question_text_expectation($essay),
+                $this->get_does_not_contain_feedback_expectation());
+
+        // Simulate some data submitted by the student.
+        $this->process_submission(array('answer' => 'This is my wonderful essay!', 'answerformat' => FORMAT_HTML));
+
+        // Verify.
+        $this->check_current_state(question_state::$complete);
+        $this->check_current_mark(null);
+        $this->check_current_output(
+                new question_contains_tag_with_attribute('textarea', 'name',
+                $this->quba->get_question_attempt($this->slot)->get_qt_field_name('answer')),
+                $this->get_does_not_contain_feedback_expectation());
+
+        // Finish the attempt.
+        $this->quba->finish_all_questions();
+
+        // Verify.
+        $this->check_current_state(question_state::$needsgrading);
+        $this->check_current_mark(null);
+        $this->assertEquals('This is my wonderful essay!',
+                $this->quba->get_response_summary($this->slot));
+
+        // Try to process a an invalid grade.
+        $this->setExpectedException('coding_exception');
+        $this->manual_grade('Comment', '10.1', FORMAT_HTML);
+    }
 }
diff --git a/question/behaviour/rendererbase.php b/question/behaviour/rendererbase.php
index f8a1d4812d892..c6c69bad13d4c 100644
--- a/question/behaviour/rendererbase.php
+++ b/question/behaviour/rendererbase.php
@@ -68,8 +68,6 @@ public function feedback(question_attempt $qa, question_display_options $options
         return '';
     }
 
-
-
     public function manual_comment_fields(question_attempt $qa, question_display_options $options) {
         $inputname = $qa->get_behaviour_field_name('comment');
         $id = $inputname . '_id';
@@ -126,14 +124,8 @@ public function manual_comment_fields(question_attempt $qa, question_display_opt
                 'name' => $markfield,
                 'id'=> $markfield
             );
-            if (!is_null($currentmark) && $qa->manual_mark_format_is_ok() ) {
-                $attributes['value'] = $qa->format_fraction_as_mark(
-                        $currentmark / $maxmark, $options->markdp);
-            }
-            // We want that the wrong entry is shown.
-            else if (!is_null($currentmark)){
-                $attributes['value'] = $qa->get_submitted_var(
-                    $qa->get_behaviour_field_name('mark'), PARAM_RAW_TRIMMED);
+            if (!is_null($currentmark)) {
+                $attributes['value'] = $currentmark;
             }
             $a = new stdClass();
             $a->max = $qa->format_max_mark($options->markdp);
@@ -153,10 +145,12 @@ public function manual_comment_fields(question_attempt $qa, question_display_opt
                 'value' => $qa->get_max_fraction(),
             ));
 
-            $error = $qa->get_validation_message();
+            $error = $qa->validate_manual_mark($currentmark);
             $errorclass = '';
-            if ($error !== ''){
-                $erroclass = 'error';
+            if ($error !== '') {
+                $erroclass = ' error';
+                $error = html_writer::tag('span', $error,
+                        array('class' => 'error')) . html_writer::empty_tag('br');
             }
 
             $mark = html_writer::tag('div', html_writer::tag('div',
@@ -172,7 +166,6 @@ public function manual_comment_fields(question_attempt $qa, question_display_opt
                 array('class' => 'fcontainer clearfix')), array('class' => 'hidden'));
     }
 
-
     public function manual_comment_view(question_attempt $qa, question_display_options $options) {
         $output = '';
         if ($qa->has_manual_comment()) {
diff --git a/question/engine/lib.php b/question/engine/lib.php
index 9e3af7f54c59e..e2164cfc928de 100644
--- a/question/engine/lib.php
+++ b/question/engine/lib.php
@@ -131,23 +131,6 @@ public static function set_max_mark_in_attempts(qubaid_condition $qubaids,
         $dm->set_max_mark_in_attempts($qubaids, $slot, $newmaxmark);
     }
 
-    /**
-     * MDL-51090
-     * Validate that the manual grade submitted for a particular question is a
-     * float.
-     *
-     * @param string prefix as constructed in is_manual_grade_in_range.
-     * @return bool whether the submitted data is a float.
-     */
-    public static function is_manual_grade_float($prefix) {
-        $val = optional_param($prefix . '-mark', null, PARAM_RAW_TRIMMED);
-        $mark = unformat_float($val, true);
-        if (is_float($mark)) {
-            return true;
-        }
-        return false;
-    }
-
     /**
      * Validate that the manual grade submitted for a particular question is in range.
      * @param int $qubaid the question_usage id.
@@ -156,14 +139,12 @@ public static function is_manual_grade_float($prefix) {
      */
     public static function is_manual_grade_in_range($qubaid, $slot) {
         $prefix = 'q' . $qubaid . ':' . $slot . '_';
-        if (!self::is_manual_grade_float($prefix)) {
-            return false;
-        }
         $mark = question_utils::optional_param_mark($prefix . '-mark');
         $maxmark = optional_param($prefix . '-maxmark', null, PARAM_FLOAT);
         $minfraction = optional_param($prefix . ':minfraction', null, PARAM_FLOAT);
         $maxfraction = optional_param($prefix . ':maxfraction', null, PARAM_FLOAT);
-        return is_null($mark) || ($mark >= $minfraction * $maxmark && $mark <= $maxfraction * $maxmark);
+        return $mark === '' ||
+                ($mark !== null && $mark >= $minfraction * $maxmark && $mark <= $maxfraction * $maxmark);
     }
 
     /**
@@ -887,8 +868,9 @@ public static function int_to_roman($number) {
     /**
      * Typically, $mark will have come from optional_param($name, null, PARAM_RAW_TRIMMED).
      * This method copes with:
-     *  - keeping null or '' input unchanged.
-     *  - nubmers that were typed as either 1.00 or 1,00 form.
+     *  - keeping null or '' input unchanged - important to let teaches set a question back to requries grading.
+     *  - numbers that were typed as either 1.00 or 1,00 form.
+     *  - invalid things, which get turned into null.
      *
      * @param string|null $mark raw use input of a mark.
      * @return float|string|null cleaned mark as a float if possible. Otherwise '' or null.
@@ -898,7 +880,13 @@ public static function clean_param_mark($mark) {
             return $mark;
         }
 
-        return clean_param(str_replace(',', '.', $mark), PARAM_FLOAT);
+        $mark = str_replace(',', '.', $mark);
+        // This regexp should match the one in validate_param.
+        if (!preg_match('/^[\+-]?[0-9]*\.?[0-9]*(e[-+]?[0-9]+)?$/i', $mark)) {
+            return null;
+        }
+
+        return clean_param($mark, PARAM_FLOAT);
     }
 
     /**
diff --git a/question/engine/questionattempt.php b/question/engine/questionattempt.php
index 26842eec64e68..0052dd1155b62 100644
--- a/question/engine/questionattempt.php
+++ b/question/engine/questionattempt.php
@@ -49,10 +49,10 @@ class question_attempt {
     const USE_RAW_DATA = 'use raw data';
 
     /**
-     * @var string special value used by manual grading because {@link PARAM_FLOAT}
-     * converts '' to 0.
+     * @var string Should not longer be used.
+     * @deprecated since Moodle 3.0
      */
-    const PARAM_MARK = 'parammark';
+    const PARAM_MARK = PARAM_RAW_TRIMMED;
 
     /**
      * @var string special value to indicate a response variable that is uploaded
@@ -651,13 +651,12 @@ public function get_mark() {
      * This is used by the manual grading code, particularly in association with
      * validation. If there is a mark submitted in the request, then use that,
      * otherwise use the latest mark for this question.
-     * @return number the current mark for this question.
-     * {@link get_fraction()} * {@link get_max_mark()}.
+     * @return number the current manual mark for this question, formatted for display.
      */
     public function get_current_manual_mark() {
-        $mark = $this->get_submitted_var($this->get_behaviour_field_name('mark'), question_attempt::PARAM_MARK);
+        $mark = $this->get_submitted_var($this->get_behaviour_field_name('mark'), PARAM_RAW_TRIMMED);
         if (is_null($mark)) {
-            return $this->get_mark();
+            return format_float($this->get_mark(), 7, true, true);
         } else {
             return $mark;
         }
@@ -1002,9 +1001,6 @@ protected function get_resume_data() {
      */
     public function get_submitted_var($name, $type, $postdata = null) {
         switch ($type) {
-            case self::PARAM_MARK:
-                // Special case to work around PARAM_FLOAT converting '' to 0.
-                return question_utils::clean_param_mark($this->get_submitted_var($name, PARAM_RAW_TRIMMED, $postdata));
 
             case self::PARAM_FILES:
                 return $this->process_response_files($name, $name, $postdata);
@@ -1026,40 +1022,27 @@ public function get_submitted_var($name, $type, $postdata = null) {
         }
     }
 
+    /**
+     * Validate the manual mark for a question.
+     * @param unknown $currentmark the user input (e.g. '1,0', '1,0' or 'invalid'.
+     * @return string any errors with the value, or '' if it is OK.
+     */
+    public function validate_manual_mark($currentmark) {
+        if ($currentmark === null || $currentmark === '') {
+            return '';
+        }
 
-    public function get_validation_message() {
-
-        if (!$this->manual_mark_format_is_ok()) {
-            $errorclass = ' error';
-            return html_writer::tag('span', get_string('manualgradeinvalidformat', 'question'),
-                array('class' => 'error')) . html_writer::empty_tag('br');
+        $mark = question_utils::clean_param_mark($currentmark);
+        if ($mark === null) {
+            return get_string('manualgradeinvalidformat', 'question');
         }
-        $currentmark = $this->get_current_manual_mark();
+
         $maxmark = $this->get_max_mark();
-        if ($currentmark > $maxmark * $this->get_max_fraction() || $currentmark < $maxmark * $this->get_min_fraction()) {
-            $errorclass = ' error';
-            return html_writer::tag('span', get_string('manualgradeoutofrange', 'question'),
-                array('class' => 'error')) . html_writer::empty_tag('br');
+        if ($mark > $maxmark * $this->get_max_fraction() || $mark < $maxmark * $this->get_min_fraction()) {
+            return get_string('manualgradeoutofrange', 'question');
         }
 
         return '';
-
-    }
-
-    /**
-     * Validates that the manual mark parameter is a float.
-     * @return bool.
-     */
-    public function manual_mark_format_is_ok() {
-        $val = $this->get_submitted_var($this->get_behaviour_field_name('mark'), PARAM_RAW_TRIMMED);
-
-        // If it is empy, we get null (it is always the case on start)
-        if ($val===null) {
-            return true;
-        }
-        $mark = unformat_float($val, true);
-
-        return is_float($mark);
     }
 
     /**
diff --git a/question/engine/tests/questionattempt_test.php b/question/engine/tests/questionattempt_test.php
index 21f6ad9fd7eef..3469ec6f067b7 100644
--- a/question/engine/tests/questionattempt_test.php
+++ b/question/engine/tests/questionattempt_test.php
@@ -109,36 +109,6 @@ public function test_get_submitted_var_not_present_var_returns_null() {
                 'reallyunlikelyvariablename', PARAM_BOOL));
     }
 
-    public function test_get_submitted_var_param_mark_not_present() {
-        $this->assertNull($this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array()));
-    }
-
-    public function test_get_submitted_var_param_mark_blank() {
-        $this->assertSame('', $this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array('name' => '')));
-    }
-
-    public function test_get_submitted_var_param_mark_number() {
-        $this->assertSame(123.0, $this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array('name' => '123')));
-    }
-
-    public function test_get_submitted_var_param_mark_number_uk_decimal() {
-        $this->assertSame(123.45, $this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array('name' => '123.45')));
-    }
-
-    public function test_get_submitted_var_param_mark_number_eu_decimal() {
-        $this->assertSame(123.45, $this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array('name' => '123,45')));
-    }
-
-    public function test_get_submitted_var_param_mark_invalid() {
-        $this->assertSame(0.0, $this->qa->get_submitted_var(
-                'name', question_attempt::PARAM_MARK, array('name' => 'frog')));
-    }
-
     public function test_get_all_submitted_qt_vars() {
         $this->qa->set_usage_id('MDOgzdhS4W');
         $this->qa->set_slot(1);
diff --git a/question/engine/tests/questionattempt_with_steps_test.php b/question/engine/tests/questionattempt_with_steps_test.php
index b5f1d4b729e1b..a5cddbd9313f2 100644
--- a/question/engine/tests/questionattempt_with_steps_test.php
+++ b/question/engine/tests/questionattempt_with_steps_test.php
@@ -165,4 +165,20 @@ public function test_cannot_get_max_fraction_before_start() {
         $this->setExpectedException('moodle_exception');
         $qa->get_max_fraction();
     }
+
+    public function test_validate_manual_mark() {
+        $this->qa->set_min_fraction(0);
+        $this->qa->set_max_fraction(1);
+        $this->assertSame('', $this->qa->validate_manual_mark(null));
+        $this->assertSame('', $this->qa->validate_manual_mark(''));
+        $this->assertSame('', $this->qa->validate_manual_mark('0'));
+        $this->assertSame('', $this->qa->validate_manual_mark('0.0'));
+        $this->assertSame('', $this->qa->validate_manual_mark('2,0'));
+        $this->assertSame(get_string('manualgradeinvalidformat', 'question'),
+                $this->qa->validate_manual_mark('frog'));
+        $this->assertSame(get_string('manualgradeoutofrange', 'question'),
+                $this->qa->validate_manual_mark('2.1'));
+        $this->assertSame(get_string('manualgradeoutofrange', 'question'),
+                $this->qa->validate_manual_mark('-0,01'));
+    }
 }
diff --git a/question/engine/tests/questionutils_test.php b/question/engine/tests/questionutils_test.php
index e5935f404244d..ab4559c7e7e62 100644
--- a/question/engine/tests/questionutils_test.php
+++ b/question/engine/tests/questionutils_test.php
@@ -194,6 +194,7 @@ public function test_int_to_roman_not_int() {
 
     public function test_clean_param_mark() {
         $this->assertNull(question_utils::clean_param_mark(null));
+        $this->assertNull(question_utils::clean_param_mark('frog'));
         $this->assertSame('', question_utils::clean_param_mark(''));
         $this->assertSame(0.0, question_utils::clean_param_mark('0'));
         $this->assertSame(1.5, question_utils::clean_param_mark('1.5'));
diff --git a/question/engine/upgrade.txt b/question/engine/upgrade.txt
index bb773396e55c1..15d1920fb75bb 100644
--- a/question/engine/upgrade.txt
+++ b/question/engine/upgrade.txt
@@ -1,5 +1,11 @@
 This files describes API changes for the core question system.
 
+=== 3.0, 2.9.2, 2.8.8 ===
+
+1) The extra internal PARAM constant question_attempt::PARAM_MARK should no
+   longer be used. (It should not have been used outside the core of the
+   question system). See MDL-51090 if you want more explanation.
+
 
 === 2.6 ===