Difficult to say without a specification but if the quote includes everything it sounds far too reasonable. Get BG in to quote for the boiler only, I suspect they will get close to your quoted price for boiler replacement only.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!