After much work on my recent purchase of a mk5 for £25, it's finally sorted and doing 585fps in .22 with super domes 11fpe.
Here's what I did
Shaved 1mm off the piston head
Fitted standard Spring from protek
Opened transfer port to 3.2mm
Fitted steel top hat, 1st one was too heavy
0.3mm ptfe piston sleeve with cocking slot cut away
Reprofiled cocking lever ears, reduced by 0.6mm
Fitted quad ring piston seal, many thanks to Louis from Holland
Fitted hw breech seal
Usual polishing of internals including trigger parts
Glued a piece of ptfe to cylinder in between the cocking lever as it was galling badly
The gun shoots with little recoil and amazing accuracy for a junior gun, well worth the trouble
Luckily mine has a tight breech with good lock up.
Many thanks to Louis for his help,