The old style bayonet filler is notorious for leaking, it rusts inside preventing the o ring from sealing, I have tried to fix these but they are not worth the trouble, just replace it and also the dowty washer behind it. The pressure gauge is also sometimes at fault, check the little vent hole on the side of it. or as was mentioned above the very thin section o ring 6x1mm under the gauge mounting block. These are the easy fix external leaks, a leak anywhere else will require the rifle to be stripped.
Use some bubble solution to help you find the leak, 50:50 washing up liquid and water.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.