This looks quite like the drone-fly in Chinery but, as is often the case, "there are several similar species" so I better stick with Eristalis.
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Thanks for the correction, people. The abdomen markings are v. similar but I've looked at another photo (with less glare) and I see what you mean about the thorax pattern.
Lat/Lng: 52.0276, -0.4681
OS grid ref: TL051376
A pocket-handkerchief nature reserve with grassland, ponds and marsh, run by the Greensand Trust in Maulden, Beds.