Well I think Ive got it sorted. I went through all the washers and it seems some had collapsed a bit, varying from 1.01mm to 0.87mm. So chose the thickest 12, still in the factory order )()()()()((( and bingo 800fps, one washer on the hammer spring and a tweak on the transfer port screw and its a pretty steady 780 +/_ 5fps measured at 110bar fill and.then at 200bar. I think the hammer spring is a bit weak though as without the washer the fps drops to 730. Time will tell wether it stays fixed.
Id like to make up a reg tester for it though, any idea what the thread is at the firing valve end?