I had a decent gas combi boiler fitted about a year ago for about £1200 all in (corgi registered local plumber called Mik on a recommendation). We chose one from a shortlist of Glow Worm, Worcester Bosch & Potterton if I recall correctly - I'll have a look when I'm home
We had a new Worcester floor standing boiler fitted last year. Not cheap but excellent quality. There is an article in the current Which magazine about boilers, well worth a read, with Worcester coming out as best buy. For a fitter, look at check-a-trade, as all tradesmen have online references, so you're unlikely to get a cowboy.