Do you have a British Gas maintenance contract?
If yes and the idea to replace the boiler came from them, then ask for a system replacement. BG has a process which is designed to reduce their costs when they have contracted to fix aging boilers which are getting very costly to them.
If the old boiler was costing them a fortune, then the local techies, ( I won't call them engineers, any more than any of you so called are engineers, are "engineers" unless you are chartered. The the rest of Europe they appreciate the difference and it really matters), then BG will be pleased to minimise its costs by installing a new boiler for only the cost of the boiler alone and BG will forget all the installation & labour costs and incidentals!
They did mine for the cost of a new Glow-Worm 30Kw with new filling loop and other extras for well less than =A3800, all up inc VAT! BG got a good deal on the boiler as well.
I'm only honour-bound to go on paying the =A3100+ annual maintenance charge while BG is protected by the two year warranty from G-W.
I know that this was a good deal and will let BG take the two year's fees of =A3200 or so while the boiler its on a two year warranty!
Everyone trashes BG, often with very very good reason, but I can only spell out the facts.
However...... they were contracted to maintain/repair a Potteron Envoy, which I suspect could bankrupt many nations in a few more years, leave alone BG!
So, if you have a BG contract, try it. Else =A31,400 on a like for like replacement is the best you'll get.
Good luck..... EP