I might just have sorted this......It suddenly dawned upon me that the auto release had been re-assembled incorrectly - the circlip had been put back on the wrong side of the screw housing. With this bit in the right place, the rear part then turned/locked as per the instructions. I also added a weak spring to hold the ball in place against the inner seal. It still leaked however until I put a stiffer spring in next to the ball and now it seems to be gas tight, fingers crossed. If it hasn't lost the gas over night, that may be job done.