Fly with rounded thorax and tapering articulated abdomen.
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I agree that this is in genus Rhagio, and it looks as if it just might be the very rare Rhagio annulatus. The common R. tringarius is similar, but tringarius should have yellow humeri (front corners of thorax), is usually a bit paler overall, and is a species of marshes and wet meadows, whereas the few records we have of annulatus are predominantly from woodland edge.
However, unfortunately I don't think we're going to be able to confirm this 100% from the photo. Some of my specimens of tringarius do have darker than usual humeri, and the photo doesn't show quite enough detail to check some of the other characters.
If you get the chance to visit the site again, and are lucky enough to see it again, it would be great to have more photos. Would ideally need to see the palps (part of the mouthparts) so a view from the side or slightly underneath would be needed. The coxa at the base of the front legs would also be helpful. There are also differences in the colours of the hairs on the abdomen, but that is hard to show in a photo. Or if you were able to send a specimen that would allow me to confirm it for certain.
See couplet no. 7 here:
Entomologist and biological recorder
here's another one that doubt has been cast over:
Could this be another candidate for annulatus?
latest pics and diptera videos
Have replied on the other observation.
(and Jeremy), this photo was taken in a meadow margin with woodland. Margin dominated in this spot particularly by blackthorn with bramble, dog rose, and some field maple and grey poplar saplings. The area is Gault clay and the meadow is ocassionally somewhat wet but only in a few places. There is a stream and one domestic garden with large pond within a 30 meters. From what you write it sounds not wholly implausible that both species you name could be present. I visit this site regularly and will continue to post invertebrates. If I find Rhagio resembling this or with any of the features you describe, I'll reply to specifically to your (Martin) message above. Thanks again, Paul
Lat/Lng: 50.9166, -0.149
OS grid ref: TQ302146
In meadow/woodland margin among brambles, nettles, dog rose and overgrown grasses.