Thanks Wally. I think it was the magazine. I played around for ages this morning looking at what was going on. Rounds were even falling out of the mag and out of the bottom port when I pulled back the charger handle and held it to the rear. When the action went forward the rounds were being stripped but not seating properly in the hole on the bolt face. Anyway, I altered the feed lips a little at a time and it seems to consistently feed, chamber and eject all my snap-caps (a run of 8) when I quickly cycle the action now. I just need to try it at the range.
My magazine was loose when I moved house and must have been damaged somehow.