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.