Dosta , many thanks for the answer. Sounds right to me although the whole system is only 3 years old so I cant understand how the radiators could be causing the problem. Is there anything in the boiler that would cause this, such as a pump not working properly. I have had a heating engineer look at this 2 times now and he has not found anything. It looks like I will have to deal with this myself now rather than pay good money for nothing.
AL
September 2008