Finally found a way forward for replacing the now extinct 53 piston head.
Tried the TR Robb ptfe version....or at least my own version ....it works fine but is a touch slammy.
Got a few more FPS ....hardly needed any lube and quite consistant. Harvey on here reporting similar but i wasnt satisfied.
Decided to order a neoprene parachute washer available on intended as a replacement for the HW25.
Its in at 22.2mm dia. but the 53 requires 22.7mm to seal.
Dried fired twice and put about 80 shots through her. Velocity went up in a slow curve ....from an initial drop from 467 fps (leather seal unit and my tune kit onboard) to 433 fps then rising back up to 458 fps after the run in described.
I removed the piston and inspected.
The seal lip was now just in full contact with the piston wall.
It may already be ideal...or could go on to expand further ...slowing, or increasing vel...
Will report back in a week.

The new head required machining of a new steel key hole piston head unit which turned put pretty simple in the finish. No screws i just used the std type flange used on the hw45 machined onto a TR RObb type head he uses for his ptfe ones...
Accuracy great.