Black and yellow fly approx 10mm.
No interactions present.
It is definitely not E. corollae, as the yellow spots do not reach the margin of the tergites.
It almost certainly is E. luniger, for the same reason. However, putting a specific name to hoverflies without a specimen is not always sensible.
My Flickr photos...
Yep I spent ages reviewing it in retrospect - Eupoedes sp This a Male - Male and female are quite variable having different marking on the abdomen; males have square commas on tergites 3 and 4, whereas females have narrow commas.
Its likely as you say ian one of britains great unsolved mysteries :-)
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Lat/Lng: 51.8, -0.8
OS grid ref: SP8314