Thanks for the information. I checked out the header tank today and found it had a lot of sediment and a brown type coating on the bottom of the tank. I cleaned it all out plus the pipework up to the gate valve and hope it has not blocked a bend as the pipe nears the water tank. I bled the radiators only as I can't find any more bleed points and even with the system running, the water seamed to come out slowly. I will have another look tomorrow and see if I can get some more air out. I am sure your information will sort things out.