Flew off before I could get better photos or net it for proper ID. Is it doable from these pictures?
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In Helophilus, the eyes don't meet in either sex. Males have the eye margins parallel atop the head, in females the margins are divergent.
My Flickr photos...
Lat/Lng: 51.546948873781, -0.32630681991577
OS grid ref: TQ161844
Horsenden Hill nature reserve (http://www.ealing.gov.uk/services/leisure/parks_and_open_spaces/find_a_park_or_open_space/perivale.html)