Just to update on this rebuild, Thanks to the cracking lads at Impact Airguns this fenman is now back up and running and smooth as Silk. Impact have really pulled all the stops out to find me the bits I needed and I can't thanks them enough.
Oh and this rifle with the 27mm piston is very smooth

WORD TO THE WISE ..... If you do need Theoben gas ram parts they will surely be able to provide what you need, however you need to be very sure of sizes etc as there were definitely different parts used through the earlier years of production. So strip and measure the parts carefully before you order.