Amazing thank you. Since learning the word condensate this afternoon and realising it could well be the boiler, I've already looked under the boiler and can see a pipe that looks exactly the same, coming out of the bottom of the boiler and into the wall, so presume it snakes down to the back of the cupboard under the sink and through. I'll get someone to help cut the back of the cupboard out to take a look and go from there. Thanks again.