Images 3 and 4 were obtained on 31/07/2013 but appear to represent the same species as in Images 1 and 2 .
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Hi Grant, I think the second two images are Soliera, you could post them separately
Edit: Dexiosoma also has red legs, they look longer though, and it's much much bigger, like a pale Thelaira, and like that fly, reliably there again the next day - so presumably territorial
Edit: you'll see Dexiosoma on bracken, it's a bit like a pale woodland version of Thelaira, I suppose, but I've not seen either on Hogweed
latest pics and diptera videos
Thanks very much for these comments
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707