I have a renter that a boiler service revealed an otherwise unknown duff PCB that managed to stop the boiler working after tampering. Gas bloke quoted about £150 and a week wait for one. I had him whip it out, I took it to a pal of mine that mends TV's, he was all over it with a magnifying glass, replaced a gizmo on the board, re-sweated two dry joints, charged me a tenner (mates rates), gas bloke refitted it and the boiler fired up.

I have two other renters with Bosch Worcester combi boilers in that I got second hand in the 90's and they sail the gas check each year (touch wood).