Thanks for such an in-depth reply! I have, however, already left it with a tech. Much to my relief, he doesn't think the neck is twisted, it's just badly in need of a set up. It was in a bad way when I left it with him so fingers crossed that is the case.
Your method makes perfect sense, I wish I'd have known about it before. As I said earlier, I'm just a little reluctant to touch the truss rod. I'll be braver next time!
Another thing I didn't mention; after the guy in the guitar shop had worked on it, the high E was catching between the fret and the binding, particularly on the 2nd & 3rd frets. The tech I've left it with said this was because the neck was too straight but that it can be a problem with guitars with bound necks. It wasn't an issue before the guy in the guitar shop adjusted the truss rod. Has anyone experienced this?