Thanks for the reply, the boiler is in my static caravan, I will be there tomorrow and will check relief pipe for water discharging as you suggest. I am sure it will be the valve as there is no obvious leaks elsewhere in the system and the pressure drops so quickly.
As the pressure is zero will I lose much if any water from the system when I unclip the valve? it is filled with an anti-freeze solution as the caravan is empty and unheated during the winter months with the DHW drained.
Alan
September 2011