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Great, thanks Jan. Yes, looking at the UK species list for Scaptomyza last night, I had wondered about S.flava.

There are only 7 on UK list :
S.pallida, S.griseola, S.graminum - seem to have grey coloration.
S.adusta, S.vittata - no records or only 1 record on NBN

Leaving S.trochanterata or S.flava as the likely candidates.
S.flava being the only one with known distribution in area.

On Boldsystems, only S.flava has examples with this pale coloration too - with dark cerci, as noted.