An ichneumon wasp, possibly an Agrypon sp. (Hymenoptera: Ichneumonidae). Just a tentative identification based on similiarity with photos here and here. The black 'lumps' are caterpillar droppings, presumably dropped by the caterpillar that ate the hole in the oak leaf. I suspect that the caterpillar might have been hiding underneath the leaf and that I had disturbed the ichneumon as it was checking out a potential host for its eggs.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-05-17.