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Many thanks to you both for the extra info on C. marginta/us/femorata. Sounds like some taxonomic revision is needed for this Genus/Species to bring some clarity and hopefully the German work will do that. The current taxonomy seems more like a pig's ear than a sore to me at the moment! These links suggest that the status of C. marginatus / C. femorata is currently undecided in the uk.
http://www.dipteristsforum.org.uk/t3261-Request.html
http://www.dipteristsforum.org.uk/t1027-Choerades-marginatus%3F.html
The location was a deciduous woodland ride, and the facial hairs do look white not yellow, so C. femorata does seem the most likely true ID and that name needs to be added to the UK list!