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I am using the key published by Brian R Pitkin in 1988, Lesser Dung Flies DIPTERA: SPHAEROCERIDAE, Handbooks for the Identification of British Insects Vol 10, Part 5e, RES of London.
First, the key to subfamilies. I think the appended photo covers the characters needed to say this is in the Copromyzinae subfamily. The long scutellar marginal setae are gone, but I think we can see one resting on the abdomen, and the bases of 4 of them are visible.