Thanks for your quick response Tony. I will try bleeding the system again (I have been doing this) before going ahead with 'sludge busting'. Over a period, I have changed a couple of ground floor radiators, and there was only a small amount of black water in the bottom. Would sludge...
Our Glowworm backboiler serves ten radiators. Sometimes it fires up and stays on, at other times it cycles on and off in quick succession (10 - 45 secs) perhaps a dozen times before it stays on. The pump is set to max flow rate, there are 2 Honeywell motorised valves, both seem to be working...