My first guess was a species of sucking louse Anoplura, which would be a new order for me. But I think not, and now I have absolutely no idea what it is! Photo and more details here:

http://www.brc.ac.uk/psl/photo/sucking-louse-anoplura

Edit: so the answer is it's a juvenile mite. Juveniles in some species have only three pairs of legs, not the four pairs they will have as adults. What sort of mite I don't yet know, maybe Trombiculoidea (harvest-mites, chiggers, etc.). Thanks to Brian Nelson, Joe Parker and Hans-Erik Wanntorp for the answer.