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It is not Polietes, and not even Muscidae. Maybe Tachinidae.
Generally, I think it is unwise to try and give a specific ID for most flies, unless you have previously run them through a key.
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I was guided by the illustration of Polietes lardarius in Chinery (Insects of Britain and Western Europe,) which it seemed to fit exactly - also a photographic illustration, (I forget where,) which appeared identical.
If it matched, Chinery is wrong. Your fly shows a clearly angled discal vein. Polietes (and there are several species which can only be separated with a microscope) has a more or less straight discal vein. Polietes is also a completely different colour to your fly. See http://www.diptera.info/photogallery.php?photo_id=2253.
some workshop notes on this image - Densely hairy eyes, cell R5 shape very elongate at point, pale scutellum, basicosta is pale, shape of last abdominal segment suggesting female. Probably tessallans or picta.
L. picta is a recent species (Belshaw's revised key), spreading throughout the south-east so this location is good.
Here's my Linnaemya v. from late summer, showing some of the same features http://www.ispot.org.uk/node/292262
Tachinids rule, etc
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Lat/Lng: 51.4, -0.03
OS grid ref: TQ371686