There's a London based company with an offer of a combi boiler swap for a fixed price of £975 (includes boiler). Similar price to what you're looking to pay just for the boiler.
I don't know this company, I haven't and I do not know anyone that has used this this company, and I'm not recommending this company - just what a quick Google search threw up - http://www.gumtree.com/london/19/39178319.html - however it doesn't seem to state what boiler they'd install - I would post again if you find out what boiler it would be, so we can offer advice.
Don't get an Alpha, they're rubbish. You can get much better boilers for a similar price (some cheaper), I think for labour only you'd probably be looking at around £300.
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!