Hi John,
Thanks for that. I checked the pressure vessel and it was empty (well atomospheric pressure) so I pumped it up to 0.8bar and it's fine now - the needle hardly moves when you draw hot water. The leak turned out to be muck or something in the pressure relief valve. I blasted it through with hot water from the heating system and it has sealed itself. I had visions of a new boiler but it cost me nothing in the end and gave me something to do on boxing day.
gary
December 2007