Gregm, it sounds like your tyre dealer is set to give Cooper's warranty a workout!...
From research before I bought, I knew the pulling-left was a possibility. I had a bit of a laugh about it and the dealer did a decent job of fixing it. I haven't had a new car needing an alignment before. I wouldn't really expect a dealer to be set up for it any more than a local tyre shop, and maybe even less experienced at performing it.
I hadn't come across many cases of rear alignment issues in my searching, hence my post, although there's a current case on a US site of a guy with similar symptoms to mine. He's getting all sorts of recommendations including "it's just how they are"... It could be a more widely spread problem, but is being mis-diagnosed. Anyway, one solution is on the record now.