i'll assume the timer is working and the boiler stat is turned up. first thing to check is the overheat thermostat, it's located on the underside of the boiler. look for a small round hole on the plastic casing, there will be a small red button with a raised cross on it, push it in to re-set. if that's what the fault is you should have a qualified engineer take a look a try to find the cause of the overheating.
Hpoe this helps.
Ian, September 2009