Central Heating Not Firing Up?

My Ravenheat Combi Boiler is not firing up on the CH - the hot water works fine. I have checked the room stat thats OK so is the controller/timer, I'm a qualified sparks so I know what I'm doing in that dept !

the valve is in mid position so seems ok.
As the gas is firing up to heat the water I would assume it can't be the air switch or fan - and the pumps working OK as its pushing hot water from the boiler to the tank.

Can it be the PCB ? or can it be the diverter valve ? if so can anyone tell me how to check it please

I'm not corgi reg so I wouldn't dream of going near any gas bits - but any help or advice would be appreciated.

Thanks