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I've been trying to key out this fly to family, but have got stuck, any help would be much appreciated. It's small (c. 3mm long), and has an attractive bright yellow face, with blue-grey dusting elsewhere on the head. The abdomen has become detached from the specimen and is not shown in the photo (I do still have it though!). Caught last weekend among grass at the edge of a sandy beach in Cornwall, south-west UK.