Why is there hot water but no heating?

I have a glowworm micron 40ff boiler,when hot water is selected on programmer the boiler fires up,but before that there is a gurgling sound like bubbling. when i select central heating on programmer the light comes on the programmer but the boiler does not fire up or even try to ignite. is it the thermistors, high limit stat or three way diverter valve next to cylinder that is stuck on hot water mode?please help me?