and that the fault is probably with the gas value which will cost
£150+ and labour cost to replace.

I'd replace the thermocouple first and then see if it fixes it. A
thermocouple is about 3 quid and is usually fitted very easily. I don't
specifically about the Baxi. It is fail safe. If you don't fit it right, the
boiler won't light.

I wouldn't replace the gas valve on it. It'd be like putting a new engine
into an 'A' reg car.

Christian.