From a walk around the lakes yesterday afternoon: my first Tachina lurida (Diptera: Tachinidae). A rather teneral female that must have just emerged from its host, the host probably being the larva of a Noctuid moth. The black lump on the fly's face seems to be the ptilinum which it inflated to break out of its pupa (which was probably itself still inside the empty shell of the host).
Identification confirmed by Chris Raper here at Diptera.info.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2012-04-27.