From a week or two ago, Valenzuela flavidus, a barkfly (family Caeciliusidae, order Psocoptera). When I was taking the photo, I didn't actually see the nymphs and the nursery. I am not sure if the nymphs are the same species as the adult, but they do seem to be the right shape for young barkflies (see here).
A couple of month ago I photographed what I thought might be a barkfly (see here), but I have recently identified that as a lime aphid.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-08-31.