I spoke with Steve Pope earlier today and all I can say is what a tremendously helpful guy and I couldn't recommend him enough.
Anyway we had a chat and fired some ideas about and after running through a few things it was highly likely that the rifle was over sprung. So basically there must be an additional washer/top hat still in the piston from a previous owners mods. After Steve gave me the internal measurements from an in tact Mk1 piston I got home and measured the piston from my rifle, 4mm difference.
Long story short after some persuasion I managed to release an additional nylon washer approx 3.8mm in width from inside the piston. I reassembled the rifle without any washers in front of the top hat and fired through the chrono. 762fps with 8.44's. Over the 20 shot string I put through the chrono this evening I had a low reading of 759fps and a high reading of 771fps which I don't think is too shabby seen as I'm assuming there will another running in period.
So as it stands the issue looks resolved however another spanner may well appear because well, it's an airgun!
Thanks for all the input gents and fingers crossed other commitments don't stop me from being able to shoot the damn thing.
Pain is temporary, glory is forever!
Death is God's way of letting you know you failed P. Company.
Last edited by kieran turner; 3rd October 2016 at 07:51 PM.