My daughter was paying quite a large monthly payment to BG for one service per year on an ordinary central heating boiler. They told her that they could not repair it if it failed as they couldn't get the parts. They also kept wanting to quote for a new combi boiler. (�3000 ??)
I have a good, reliable Corgi-registered plumber and sent him round to see her. He said that he could get the parts if necessary and could also supply and install a new combi for a lot less than BG. This plumber wouldn't recommend having a combi boiler and his yearly service costs a lot less than the monthly instalments to BG.
I would suggest that you get a second opinion from a reliable Corgi-registered plumber.