Here's some code to make a Raspberry Pi control a Tower RFWRT wireless thermostat. It's adaptable to other makes and models, but this is the one I've got. More info about reverse engineering the Tower RFWRT.
There's two bits. A C executable, called boiler_control, which when passed an
on or off argument, will send the appropriate signal to the heating control
unit.
There's a separate Go executable, called thermostat, that runs thermostat
control loop. It can be configured to read a file at regular intervals for the
target temperature, allowing the use of cron to schedule your heating.
thermostat can be cross compiled from any Go compiler, but it's probably
easiest to compile boiler_control on the Pi.