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3 April, Clitheroe England. Guessing from the Gallery, I thought the pre-sutural dcs too strong for Tephrochlamys, so maybe Heteromyza? Help please, Andrzej!

I caught a pair of these (possibly not this pair, as there were several flies together on the birch tree - they seemed to be finding sap), so what should I look for to determine species? Is there an easily available key?