A leaf of a hawthorn bush (Crataegus sp) that has been badly discolored and distorted by galls.
Turning the leaf over revealed the galls to be full of black aphids which were being tended by an ant:
I think it probable that the aphids actually caused the galls, and that they were not just taking shelter in pre-existing galls. My main reason for thinking this is that I could find no other galled leaves nor any aphids on the rest of the bush nor on any of the neighboring ones.
Photos taken in Whiteknights Park, Reading, UK, on 2014-05-05.