Larvae and eggs of the green dock beetle, Gastrophysa viridula (Coleoptera: Chrysomelidae). Identification based on photos here and here. The larvae are demolishing this leaf so effectively that I don't think the larvae still in the eggs will find any left when they hatch out. For photos of the adults that I took last year, see here and here.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-15 and 2010-06-19, respectively.