myson apollo boiler fire up?

I have a problem with my boiler on subsiquent firenup.when boiler first kicks in on timer water heats up and heating kicks in.when temperture reached on both cuts out.when temperature drops either on hot water or heating the boiler does not fire up again.thermostats chgd and programmer chgd but still no joy.can you please help.

trevor ayliffe
October 2010