Ok so more things tried.....
I part inserted a pellet into the breech and closed the underlever. The pellet was a fingernail proud( no seal fitted). The lip on the compression chamber is 2/10ths of a mm......and it's still the original cocking arm as the rivit is unmolested.....I add this IS an early gun at 1291***.....surely it's not me being picky.....
With the lever pin out and a transfer punch in place.....no probs, but that's fractionally under size and surely not right....
Cheers for all the advice,