Sounds like you need a new expansion vessel. To confirm this, have a look outside where the boiler is. There should be a 15mm copper pipe coming through the wall about 3 feet below the flue pipe (normal istallation) if this pipe has evidence of water loss (ie dripping) then you probably need a new vessel inside the boiler or the safety valve could just be stuck.(CAUTION THIS WATER MAY BE EXTREMELY HOT) If no evidence of water is present then you have a leak in the system somewhere - if this is a upstairs leak then you will see water patch on downstairs cieling soon enough.
I would say most likley vessel as 20mins is very quick for pressure drop.
Stewiesnake, March 2010
