To the untrained eye, rather like a scorpion fly
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It is probably the Downlooker Snipefly, Rhagio scolopaceus. There are two British species with patterned wings, but the other is much scarcer and I have never seen it. The snipeflies are covered by the book British Soldierflies and their Allies by Alan Stubbs and Martin Drake.
I agree with Richard, this looks good for Rhagio scolopaceus. Although the daignostic features that separate it from R. strigosus are not visible in the photo it would be very early for that species which Stubbs and Drake give as early July to early September.
Title changed in acceptance of this identification.
In years past I was lucky enough to live near colonies of the scarcer species, Rhagio strigosus, in Oxfordshire and Berkshire. It started flying in about the second week of June, becoming more frequent in July and August - in good spots it could then be seen in hundreds. However, as far as I know recent records have all been near the chalk in Berks, Oxon and Surrey. It's never safe to rule out a species on the basis of distribution alone, it might expand its range one day, but I think Rhagio scolopaceus is by far the more likely species at this location on this date.
Entomologist and biological recorder
Lat/Lng: 50.4, -4.3
OS grid ref: SX4268