Unless you had a really, really crap boiler you'll be moving from something with around 60-70% peak efficiency (common say 20 years ago) to one with a 'claimed' 90+% efficiency.
But does it really save anything?
Taking my current gas bill with a condensing boiler (SEDBUK 'A' 90.7%) fitted about five years it would require about seven years to payback the current capital cost of a replacement boiler, totally neglecting any financing charges or costs associated with installation - and that's with just cavity wall insulation, about 8 inches of fibreglass in the loft and no double glazing, 450ft above sea level on a windswept hill in the freezing north of England.
Also just a boiler change doesn't always achieve the improvements often claimed. Unless you are prepared to seriously compromise on room comfort with lower temperatures, or your radiators were oversized to start with then with a condensing boiler you'll need much bigger radiators fitting (or singles changed for doubles)
This is because to gain all the benefits of a condensing boiler the return water temperature (water that has gone through the radiators and is about to enter the boiler again for reheating) has to be as low as possible, other wise your boiler will not be operating in the condensing region - basically a bigger proportion of the waste heat will still be exiting via the the waste flue.
Previously you'd have designed radiator sizes for around a 90 deg C feed 70 deg return, a condensing boiler ideally needs 75 deg C feed, 65 return (or less if possible) 45 deg C return temps are about the best you can ever get, and for these you need massive radiators such as found in underfloor heating (or thermal stores).
Of course if you get boiler installed by the likes of British Gas or the a Warmfront contractor you'll *never* achieve savings that repay even the capital costs let alone the costs of financing. The sooner more money for improvements goes very significantly improved levels of insulation or total housing stock replacement the better. Wasting it with administration and vastly inflated installation costs is a disgrace.
But the fundamental problem now is the UK has a massive private housing and nearly all the public sector housing has been sold off and is in 'private' hands. Rebuilding and replacing the current housing stock isn't really going to happen. Ever. You'll certainly see 1950's housing around in 2050, almost unchanged and leaking heat like a cave.