In the end I gave up on DSM. Got there at 10.45am and got to the back of the queue. When I'd queued till about 1pm the man from Apple with the scots accent and beard (I asked if he was Francis, he wasn't) said that they only had 38mm left. I want a 42, so I left. I rang them back just now and the next delivery is Wednesday.
So, a somewhat wasted trip, but I didn't want to buy something I wouldn't be happy with.
Oh, and I have never seen so many Scalpers in an Apple queue. Seemed very few genuine fanbois to me, but I guess that's London for you.