If you start to strip the pistol and get the endplug/ guide off, check for scoring/gouges in the cylinder. I have seen a few Webley pistols that have had modern lubes in them that the piston starts to dig into the cylinder on cocking. I did a Premier Mk2 for a bloke last year that needed a small rotary tool and a file to clean the gouges to enable the piston to come out. I used chainsaw oil to lubricate the thing, but any thickish mineral oil should be ok. I am not saying that is the problem, but worth checkking before using brute force and doing more damage.