Peter, it seems probable your fly will be Desmometopa sordida, which has few records in the county (and probably anywhere in the country), so if you can find some again next year and send to Del for identification with record details (location, grid ref, date etc) that would be good.
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The weblog below is for naturalists to use to report interesting sightings, ask questions, report on field meetings and generally post pictures and any information or questions generally relevant in some way to the wildlife and geology of Essex. You will need to register and be logged-on to post to the forum, and you need to upload pictures first, for use in posts. Find out more