Are you sitting comfortably? Then I'll tell you a story...
BG turn up just before christmas at my sister in law's house to service a boiler that they have one of these maintainence agreements on. Oh no
says the engineer. There's a hole in your boiler here, I have to condem the boiler, you need a new one. He then proceeds to disconnect the gas supply from the boiler (by just cutting a section of the pipe) and turns the gas off at the meter, so no chance of using the gas fire in the front room.
As he leaves, he says, I'll get the office to get in contact with you to discuss a new boiler. A few days later, and still no word from BG she phones up. "Oh, we'll send you a brochure of our boilers"
Feeling thoroughly p**sed off with BG and with teeth chattering, she calls local HE
New HE turns up, and is confused
the "hole" was in fact a bolt hole that chances are the previous BG engineer had forgotten to put a bolt back in. Puts a bolt in the hole, replaces the gas pipe and hey presto, warmth
To be fair to BG I do know some people who purchased their boiler from them and are quite happy with the service. However I know many more people with older boilers that BG try to sell a new one to. I don't know if the engineers are on comission but from what I have heard, they only seem interested in selling new boilers.