My first fly from the family Ulidiidae: Otites guttata (Diptera: Ulidiidae). Sometimes known as Otites guttatus. Rather like a chunky Tephritid fly. This seems to be the only Otites sp that is found in the UK. Identification confirmed by Paul Buek here.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2012-05-06.