advice to replace boiler

Hi, We've just moved into a 4 bed / 1 bath detached house. The combi boiler is around 8 years old, it's working but when our friend who is a gas engineer serviced it recently he believes there is a problem with the heat exchanger. We're looking to extend the house in the near future, extending...

I have a Boilermate II under the stairs (in the kitchen) and a Potterton Suprima gas boiler in the kitchen and am impressed they have lasted about 18 yrs even though I live in a very hard water area! I have had a service contract with British Gas and for many years they have serviced it...