Just re stripped the TX and removed a slip washer that I had put under the top hat......It now cocks.
With a Mk 3 V-Mach spring it makes 8.5 f lbs, with the Mk2 Knibbs spring, its at 11.2 f lbs.
New seal, so will put a tin thru it and recheck.
Many Thanks for all of your comments...shame I made a school boy error
Cheers
Steve