baxi 105e gas fires up then goes out after approx 1 second?

I have a Baxi 105e combi. Problem was I was getting no gas flow to the burners. checked the pressure switch which is working fine.generating a spark ok & fan switching on OK. Local engineer said it was the PCB card. SO he changed it and now we get another fault and he's scratching his head!!
New fault is that on demand form either HW or CH the burner lights up for approx 1 second then goes out! the system then retries three times with exactly the same outcome then faults under the 30oC led (flame). its november and the house is freezing!! help will be appreciated.