maxol microturbo 402?

when i turn the hot water on from the programmer, the boiler will fire up to heat water but the central heating has started to come on as well, albeit the ch neon light is not on on the programmer? this happens even with thermostate right down, previous to this fault i had to have the hot water on for the central heating to work but had the opption of not having heating on via thermo-help any ideas?