Baxi Combi Instant 105e hot water works but no heating?!?

Hi

I have a Baxi Combi Instant 105e.

The hot water preheat function works, and so does the normal hot water function when you turn on the tap. You can hear the pump starting up, the clicking of the diverter valve, and clicking on the spark and the lighing of the flame. It then heats up the water to the desired temperature as indicated on the temperature neons. But the central heating is dead, with no reaction from the boiler whatsoever.

So if I turn the hot water at the taps, and off by turning the preheat thermostat to zero, and turn the central heating thermostat up, and make sure the timer is on 'always on', nothing happens. The only light that is on is the power light, so no reported errors, and no sound either.

I have taken the front off and examined the circuit board and I've checked continuity for the timer and it's fine, and the timer has power also. This bioler dosn't have a roomstat.

Any ideas guys? I'm stumped.