Potterton puma 80e no central heating hot water ok?

The central heating has stopped working without warning. The hot water works fine and the modulator light goes out when tap turned off. The pump works as it should after after drawing hot water and the fan is OK. However, the boiler does not ignite for CH. It is as if the timer has timed out, even if the system is on constant. I have replaced the CH thermostat but this made no difference. Any ideas?