heating works intermittently?

the timer has been set up and working fine but occasionally decides to put the light on for the central heating but the boiler doesn't fire up. if i advance the hot water the boiler fires up immediately, (therefore i don't think it's a boiler problem) i changed the thermostat thinking it was that and although it worked fine for that evening, i now have the same problem again. do i need to replace the timer unit or can i reset it somehow? thanks