I fixed it! I dismantled the lens again just now for another regrease and realised there is another ring around the helicoid which, once removed, allows the helicoid to be fully unscrewed. Gave everything a very good clean with white spirit to remove the grease, regreased the helicoid lots (I applied as little as possible but it wanted a lot to get moving nicely) and now works perfectly. Thanks for everyone's responses
. Now the only problem is a little bit of fungus and that horrible looking broken focus ring. There's also some weird rainbow blob at the very edge of one of the front elements but it doesn't seem to be a big problem.