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Thanks for the suggestion Paul! :) Yes, good one, pun :D Obviously I don't have a clue, but the shape does suggest Melanostoma. Funny though that I had found a mention of M. dubium as being an upland species, being the same as Melanostoma mellinum when searching for that, but above 460m in north and western Britain. I'm in the Midlands and around 80m at a guess. As I had one which I thought was Melanostoma mellinum, perhaps it's a melanic form of that?

I found a paper on this, I guess you have already seen it.

http://www.malloc...DUBIUM.pdf

After reading some of the pdf I see why the pun! Some specimens collected turned out to be Platycheirus sp.! :o

"With regard to the separation of female M. dubium from the not infrequent melanic form of M. mellinum, Andersson
gives the character of a divided tergite 8 in the former whilst in M. mellinum this tergite
is in one piece. This represents the first definite taxanomic character in the distinction
of the two species"

Janet