seems to be switch related to me, if it fires on demand for hot water (poor as it might be) but dont fire for demand of heating then it can really only be down to a switch fault, if youre using the timer on constant, try switching it to timed and flick all the pins out, no joy try playing around with the main selector dial (hw - 0 - hw+ch).
it might fire for you but id advise getting it looked at, like i said it could be a few things and a few of them could be stupid little things like selector spindle has rounded off so doesnt turn pcb switch........... a engineer will usually try their best to rule out rather than guess and replace. best of luck
ste
November 2009