We had 4 quotes - one from a friend in the business, 2 from local firms, and 1 from British Gas.
The British Gas one came in at 3,700 quid for a brand new Brit/Scot Gas Boiler, removal of the old tanks, and boiler, fitting all the piping, electrics, radiator valves, a powerflush, year's central heating care, removal of old asbestos flue.
The other 3 quotes were VERY close, if a little bit cheaper.
The good thing about the British Gas quote, which was extremely detailed, was that we could pay 50% up front, then pay 50% over the next 12 months, interest free.
This gave us more of our budget to spend on the rest of our project, and we had the peace of mind of it being installed by British Gas.
I would seriously consider them