If the boiler is being put in exactly the same place as the old one it is a very expensive quote. It does not take very long to take the old one out and put another in! Also Google the list price of the boiler, remembering to add VAT, as they may be putting a large mark up on that as well.
I just got a new boiler installed and the whole thing cost £2,100 inc VAT. To be honest, I thought this was a bit steep but the engineer came on the recommendation of a lot of people, who vouched for his trustworthiness. He said that it was about as cheap as it could be for a decent job and decent boiler.
I can't tell for sure if he's right, but hearing that you have been quoted £650 more than that makes me think the going rate really must be £2,000+.