A figwort sawfly, Tenthredo scrophulariae (Hymenoptera: Tenthredinidae). It is the yellow-orange antennae that distinguish this species from several other black and yellow sawflies. The larvae of this species are also rather pretty.
The red flower is, I think, water figwort (Scrophularia auriculata = Scrophularia aquatica), but the leaves are from another plant, probably bindweed. A couple of months later I came across weevils on the same figwort plants, see here.
Photo taken beside the large lake, Whiteknights Park, Reading University grounds, Reading, UK, on 2009-08-12.