Potterton Flamingo II, hot water is fine but central heating doesnt ignite?

Hello

I have a Potterton Flamingo II. The hot water option works no problem, but when I select either water+heating or heating, the selector light is on, but the boiler does not ignite and hence the heating does not work. I have flushed the system in case of air bubble, and tried to reset the whole system. But nothing! what could it be.. thermostate faulty? faulty part? please help!!