A plumber has quoted me �2500 for a new boiler, but I have just seen 1 in b&Q for �400. what's the difference?

Ok here goes, �2500 - the Vat= �326.09 =�2173.91- cost of quality materials �1000= �1173.91- say in total costs of estimating, ordering, registration fee's, use of tools equipment (power flusher, drills, gas yser, transport, insurance), labour 2 days for suitably qualified engineer + mate, environmental disposal of removed items, Guarantee =�1173.09. in all �2500 is not far out for a Professional installation. Or get some numpty to fit a piece of crap from B&Q (they are cheap for a reason), that you will have to replace in 5yrs. Hope this helps:):):)