A black Tachinid fly, probably Zophomyia temula (Diptera: Tachinidae).
About 10 metres further along the field I photographed the following which, at first, I assumed to be the same species.
This one looks slightly fatter and has more white on the thorax and behind the eyes. But I thought that these might be just male-female differences. However, what convinced me that it was a different species was the outer cross-vein at the trailing edge: here it is distinctly concave while in Z. temula it is more straight. It is a Thelaira sp, probably Thelaira nigrina = Thelaira negripes (Diptera: Tachinidae). The only other UK species in this genus is Thelaira solivaga but this is less common (see here).
Both photos taken in the field below Chazey Wood, near Caversham, UK, on 2011-05-20.