Thanks Fireman, I'm going to give him a call , and get it tested right. Given it's some 45 years old I wouldn't be surprised if the case seal was a little tired, but that seal is accessible, and so if causing a problem something could be done about it I wuld imagine. It was reference to some mysterious seal in the flue that was quoted as being the death of the thing, he mentiioned something about having to take the whole heat excahnger off the wall to gain access, but I'm not sure such a seal exists? There is a sand / cement screed making a good seal from the outside and as far as I can infer only the seals shown on my attachement. Once I have a resolution will update.