Potterton prima fires up for hot water but not for heating?

Please help My potterton prima boiler fires up no problem for hot water but yesterday it went off when the central heating had been on for a while and now it won't fire up for the c/h but still fires up for the hot water as normal. It's happened a few times before in the last few years but after messing around with the drayton timer it came back on with no other probs, this time around it's staying off!?! Hoping someone can help! (the timer is a year old and works fine)