Make sure they're CORGI registered. Otherwise if your house blow up, you aint insured. Also, make sure they're CORGI registered. Not just 'da company what they work for'.
I git mine done for �850 by a plumber who was a friend of a friend. �600 for the boiler and the fittings, �150 labour (not bad, only took him 6 hours - �25 per hour!)
I though this was a bargain, the plumber who lives across the road quoted me c�1500, possibly more.
I had a new boiler fitted a year ago and it cost �850 to buy and then �350 for labour. You do need a CORGI registered plumber and they need to come back every year to check it is OK for CO gas and a general maintenance. Well worth it though!