This is a very belated follow up to the comments from Phil, Steveb, stevetheplumber and Masood. Thanks very much to all of you. Unfortunately I've not resolved the problem yet but now have a much more urgent one. I have a hot water underfloor heating system in a 9 year old 2 bed flat. I'm very pleased with it and would never look at anything else in the future. But -- one bedroom is in deep trouble. It's permanently on, full power! In case you think this a sudden emergency I have to tell you that I am a serious procrastinator. The problem has been there since we bought the flat in Oct 2012 but since it's my bedroom, not my wife's , I just do without bedclothes until the heating can go off again. I'll write an account of my investigations so far and start a new thread as soon as possible (don't hold your breath!}. Once again thanks for your interest and I do enjoy reading other members' cliff-hanging who dunnits!