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

neil parson, January 2011