OK, they've arrived and I tried them for the club training last night. Very nice.
Much thinner and lighter than the Nike Dri-fit ones I usually get. Therefore a lot more easy to forget you're wearing them. Plenty warm enough too. They aren't as warm as my usual ones at first, but retained the heat well, keeping my legs nice and warm whenever we stopped.
Not sure I'd wear them without shorts over the top as they really are tights more than leggings (if you see what I mean).
The Medium was a nice fit and bears a pretty close comparison to what I usually get. I've never tried compression gear, but these seem tighter around my (skinny) calves in a way I quite liked.