Popped into Vines in Guildford and they turned out to be VERY helpful in the end. They took the barrel out and found it to be bent (cough no idea how that could have happened ...) and were about to order a new cylinder when I mentioned to them to check the number of 'shims' in it, it turned out to indeed have the 7th unnecessary one so they took it out and it works perfectly now. You guys were spot on. (XBX)
in hindsight I probably could have fixed it myself, but knowing my luck, if I removed the key ALL shims would have fallen out and I'd have been in a right mess - so rather they did it. Just glad it turned out to be faulty and not my blundering incompetence lol
Result! I've checked the other 7 cylinders and they all look fine.