Ask BG for a quote under their "System Improvement" programme.
If BG doesn't want the hassle/cost of repeated repair they may install a new boiler for just the cost of the boiler itself and lose the cost of the extra parts and labour.
They did it for me a couple of months go, replacing a dreadful Potterton Envoy with a Glow-worm 30hxi for £775.55 - all in inc VAT!
This included a new filling loop, new flue, plus a new pressure gauge and safety valve on the expansion vessel. He used a couple of metres of 22mm pipe and fittings. The flue was in a slightly different place from the old one, so the technician had to brick up the old hole and cut a new one.
All done in well under a day! The deal did not involve BG's salesmen but, as the man explained, it was their service dept. looking for mutually agreeable. ways of cutting their costs and my aggravation. It suited me since I had planned to get rid of the wretched Envoy this spring anyway. EP
Glad it as OK for you, I found the problem with BG is the sales people. They are trying to get rich quick so they will inflate prices based on what they think they can get away with. Never buy their warranties, no matter what scare stories they lie to customers about - if you read the terms and conditions they do not cover you anyway. They should only sell them after the first year once the manufacturers warranty expires. Even after that you still can claim from the manufacturer as products are expected to have a certain lifespan. It would be unreasonable for a product just to last a year or two! BG tend to contract out anyway, so there is a huge percentage for the sales person, then a cut for BG after they have paid a third party.