Well, today I bought a pair from NikeTown, Oxford Circus. They had a trial stand with a treadmill, and the girl who was working is a runner and seemed to know what she was talking about.
There's a few different versions. The lightest and most flexible version has a positively flimsy upper, but there are others which are less flexibly with a more durable upper.
I bought the light and flexible version for £60. Yes, there is a bit of a heel in them, but I'm willing to give them a go for around the house and walking to the local shops and so on.
I've had lots of foot problems, and to be honest, since wearing orthoses, the bottom of my feet feel dead, so these may liven things up a little. I'll report back in a week or so.