When a learned tome such as Britain's Hoverflies dumps me amidst what it refers to as "the difficult five", it's time for me to bail out at genus level.
No interactions present.
... the book did say they were difficult. ;-)
Would a larger or zoomed in image help at all?
Another view John ??
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I'm afraid that's the only angle I got before it scarpered. ;-)
I'd have thought the apparently short 1st antennal segment, plus the long hairs on the thorax of this female, would make it cautum.
My Flickr photos...
I'll file it as you suggest.
Lat/Lng: 51.901865214178, -0.67540168762207
OS grid ref: SP912234
A flooded old quarry south of Leighton Buzzard.