One of hundreds seen on the biodiversity survey carried out by Amersham U3A
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I agree with the male in the first picture, but the second (female) looks more like a Dilophus. As far as I can see, the front tibiae have medial spurs and apical spurs, rather than the single long and short spur of a Bibio.
My Flickr photos...
I've uploaded a close-up of the second fly, to see if anyone can confirm the ID
Looking at this again, the second picture is definitely a Dilophus, and almost certainly a female of the common D. febrilis, as the wings appear to be very dark, near black, with pale tips.
Lat/Lng: 51.74284, -0.62607
OS grid ref: SP949057