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Please note that I had never seen a Sphaerocerid before finding this fly in the garden. All my comments are based solely on the RES Handbook 10.5e. I have been unable to find many photos on the web to clarify some of the problems I am having to arrive at an ID that seems (to me at least) consistent with the Pitkin key.

Here is a side by side comparison of photos of the head of this specimen with the diagrams for Boborillus uncinatus and Crumomyia notabilis in Brian Pitkin's key.

Best wishes,
Martin