So today is the day for this, put a few pellets through the 95k to warm it up, and then a couple through the chrono - horrible shot cycle, and only 4.5 ft/lb!
Had to run a couple of other guns through it to double check, and they came out as expected. So must be a knackered piston seal, breech seal is looking a little flat too. Luckily I have replacements for both.
So time to get this thing stripped. Fingers crossed I get this sorted before it gets dark, otherwise it might have to sit until next Saturday for accuracy testing.