As above, the hot water works but the radiators do not come on. It's an old gas powered central heating system. We're thinking it's the wall thermostat at fault but is there anything else it might be?
Update:
I thought I should post a follow up for the sake of future Googlers.
It turned out to be the pump as ChrisF had suggested. The hot water still worked because it's gravity fed from the loft (or attic if you prefer ;)), whereas the pump wasn't kicking in at all. 20 years' worth of use had clogged it up. When the system was emptied the water that came out of it started brown and quickly went black.
With your car you get it serviced every year. Boilers get serviced on a regular basis too but the system itself doesn't, so over the years the muck builds up and builds up and nothing gets done about it causes something to break, i.e. now.
Best Answer
The most obvious thing that springs to mind is that you have air in your system.
Do the pipes between the radiators get hot, or the bottoms of the radiators but not the top?
If so then you need to bleed your radiators. You should have a little key with a square hole. Carefully open the valve at the top of the radiator a very small amount - you should hear a hissing sound of air escaping. Wait until water starts to dribble out - use a cloth to catch this. Repeat for all radiators.
For more information, pictures and a couple of videos see The Ultimate Handyman
UPDATE
Turn your thermostat all the way up - to make sure that it registers the current temperature as too cold - then the boiler should kick in. If not then it's the thermostat. If the boiler does kick in check the central heating return pipe - this should get hot after a few minutes. If not then there's either an airlock in the pipes themselves or the pump isn't working.
Can you hear the pump turning. If not there's a screw head in the centre of the pump which controls the speed of the pump (see the pictures here). Turn this clockwise. If the pump still doesn't turn you may need to replace it.
It could also be the electronics in the boiler not sending the signal out to the pump.
At this point consulting a heating engineer would be your best option.