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Hi Chas.
I never work with Asilidae yet. But I tryed with different key which you surely have not (Diptera of European Russia (I think you used "Soldgerflies and allies of British isl";), which I have, but not now).
I can tell you that I've got same result as you. My key do not ask (and do not give) hind leg colour at all.
But my key led to D.linearis both in case if mesotonum all black (as in your photo) and as well if mesotonum with red spots. So, the colour may be rather different.
The main pattern of D.linearis - 1-st segment of hind tarsus is as wide as t3 top is clearly visible at your photo.
So, at your place I'd say Dioctra linearis.
Nikita