chrisnutt,
It is a gas boiler, so I think the responses I have had to this problem are probably correct.
BTW merchant banker = rhyming slang :wink: and I have a very minor role in an investment bank in London
I have recently had an extension to my house and the central heating boiler has been moved to the garage. The flue from the boiler runs up at an angle then through the garage roof.
The problem I have is that water is dripping out of the flue where there is a bend. This only appears to happen...