I think i may well have to admit defeat and get someone in to just have a look over it. Your advice has been absolutely top drawer and I really appreciate the time you have spent trying to help, particularly when i have to ask you what most are the things are that i am supposed to be seeing!! You are a credit to these forums.
Shame im not in Hertfordshire, I would have quite happily paid you to come and have a look at it for me! I do have a few friends there and please be assured that i will be mentioning you to them, should they need a heating engineer anytime soon.
Thanks again Dan and i will give you a shout next week with the outcome.....