This is a good question.
What's the answer? No idea!
Is the boiler being changed because the old one is faulty?
Or is it being changed in order to gain greater energy efficiency?
I've been looking at doing the same myself. but it seems that the supposed 15% better efficiency comes at an uneconomical price.
It seems that it takes longer than the brand new super-duper £2K condensing combi boiler's projected lifespan to recoup the initial outlay in terms of future savings made via reduced energy bills.
Ergo - Unless the boiler lasts longer than predicted, you'll never recoup your money.
That said, Viessmann boilers (
http://www.viessmann.co.uk/en/products/gas_fired_condensing.html) seem to have a good reputation, whereas Worcester Bosch boilers are pretty crap in terms of reliability!
Valliant boilers also seem to get the thumbs-up -
http://www.vaillant.co.uk/I'd be very interested to read further comments.
TT