Large Scathophagid fly on Alder in the middle of a Carex bed.
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Cordilura picipes and Cordilura ustulata also have black legs. Arista less plumose (?). [Diptera.info images]
Thanks for that. picipes only has two scutellar bristles and ustulata has less plumose aristae, as you say. I did take the specimen, so was able to see extra details like the bristle on the side of the third antennal segment. Really smart flies. Eating an owl midge in the last picture, I think.
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Lat/Lng: 54.2804, -0.69
OS grid ref: SE853879