Have a Baxi boiler problem. Any heating engineers out there?

I have a Baxi boiler model FS 601 and the burner including the pilot keep going out. So far spend �400 with 2 different engineers and had new thermopile and gas valve been fitted. No faults diagnosed. Still the problem is NOT solved however it seems to be worst at night. Is it because of abnormally low temperatures outside? If so how does ambient temperature cause the problem? Boiler is housed in cellar with only flimsy door to the yard. Temps are tonight, minus 7 degrees.