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.