I have no qualifications or experience...
Your boiler was last made in 2002 and is 78% efficient - modern boilers are about 90% efficient.
I believe combi boilers have two heat exchangers one for DHW and one for C/H - they can vary in cost from £80 - £400 from what I can tell - plus labour costs too.
A new boiler may cost £800 and a lot more to have it installed. There may be costs associated with running a new gas pipe to the new boiler - I think they must be 22mm now rather than 15mm.
The leak may not be from the heat exchanger
Only you can determine if it's worth it
Have an engineer call and avoid British Gas unless you are very rich.
Good luck...
Peccavi, March 2010
