Yay! After much experimentation trying to decode and emulate the on/off signals from my wireless thermostat I finally cracked it this evening and now have full Arduino control of my boiler.
In the end I used an SDR to capture and record the audio from the transmitter at 433.8MHz and then loaded the .wav file into Audacity to view the pulses and work out the timings. Took me ages to figure out that a LOW output from the Arduino resulted in a HIGH output on the radio module.
The plan is to hook the controller up to an ESP8266 wifi module to allow me to remotely control my boiler when out of the house and to give better control with multiple temperature sensors around the house so I can control temperature based on what rooms are occupied rather than a single thermostat in one location.