Ok, Got new compression cylinder and tune up kit which included the piston, mainspring, guide and power washer plus 2 greases. they were bought directly from the exporter Sportswaffen-Schneider. Hoping these 16 Joule bits cure my lack of power problem!
One word...Brilliantly! All were the correct parts and at a reasonable cost. I suggest that if you want a single piston though that you buy the 16 Joule UK export kit which has lots of other bits including the new mainspring for only a £1.00 more than the single piston part.
Now running at a consistent 11.30 - 11.40 depending on pellet type.