I seem to have learned a lot about TX cylinders from this thread, as well as about sizing seals. I certainly was fooled by the cross hatching into assuming it had ben properly honed, but I noticed the tube has clearly been rubbing oppsite the shoe recess. I have no internal mike to check with, but the gun was running with only a few fps variation with the old piston and seal, so any variation in diameter shouldn't be a problem. It is now back together with a tight (but not ridiculously tight, as it was out of the box) seal, but only giving me 9.5 with 42mm preload, I suspect due to the tight seal. I'll put some pellets through it in the garden tomorrow, then see if that improves things.
I don't think I am brave enough to try Tony's idea of boiling the whole assembly - has anyone tried it?