My sisters boiler packed in a couple of weeks back, and was condemned. BG quoted her nearly £4000 for a replacement, straight swap, no work with the rads, etc. A local fitter quoted her £1300. Obviously she went with the local lad, and when he turned up, he told her that when he went to fetch the boiler, the suppliers had but their prices up, but said that he would honour his original quote.
Moral of the tale seems to be always get more than one quote.