Add advice on writing on behalf of someone else

templates/website/about-constituency.html

@@ -25,6 +25,7 @@ <h2>What postcode should I use?</h2>
 <a name="top"></a>
 <ul class="side-nav">
 <li><a href="#lookup">I know my address but not my postcode</a></li>
+<li><a href="#onbehalf">I’m writing on behalf of someone else</a></li>
 <li><a href="#services">I’m a member of the armed forces</a></li>
 <li><a href="#expat">I’m an expatriate</a></li>
 <li><a href="#bot">I live in a British Overseas Territory</a></li>
@@ -43,6 +44,25 @@ <h2>What postcode should I use?</h2>
 any UK address and it will tell you the postcode.</p>
 </dd>
 
+<dt><a name="onbehalf"></a>I’m writing on behalf of someone else</dt>
+
+<dd>
+    <p>It’s better if that person can write their own message. By convention,
+        representatives will only act on behalf of their own constituents, and
+        before taking up any case, they will always want to make sure they have
+        the permission of the person(s) it concerns.</p>
+
+    <p>If you’re writing on behalf of someone who doesn’t have capacity, the
+        ‘correct’ procedure is to contact your own representative and ask
+        them to take up the matter with the most appropriate colleague. So,
+        use your own address and postcode, and in your message, give the details
+        of the person you’re writing about.</p>
+
+    <p>This is also the case if you’re writing about, say, your office
+        premises, or another business. Use your home address and postcode.</p>
+
+</dd>
+
 <dt><a name="services"></a>I’m a member of the armed forces</dt>
 
 <dd><p><strong>How do I register to vote?</strong> As a member of the armed