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