falling pressure on baxi bahama 100?

hi can any one help me please. I have above boiler. Recently it let the water out the central heating circuit via the pressure relief valve. I charged the system again and literally watched it fall before my eyes. Went outside and the water had again let by this valve. After charging and turning the valve a couple of times i got the pressure to hold. Now 2 days later the boiler has shut off and drained down again via the pressure relief valve. This time when i charged it their was water bubbling out directly under the nipple. I checked and their is no seem. Can it be patched or does it need replacing? Also should pressure relief valve be changed at same time? Any help much appreciated.

Geoff
November 2008