How do you hold the sleeve in Mick? What are the lines on the Mk2 breech block? Has it had a block on it for a pistol scope or something?
I bought a dog rough Hawk mk3 for a fiver a few years ago (the one I filed the cylinder and end plug to take a meteor pin as it was so eggy) and that shots at 9.5ft lb with an O ring and a meteor mainspring and maybe a floating guide. It is one of those guns that people can pick up and just shoot with and it feels right.
be interested if you can try the Ptfe washer thing if you have an unmodified hawk somewhere.
I think Hawk mk ones plink well but the trigger can be heavy unless you have a nice "tired" spring.