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If this was really out to sea off the Norfolk coast, as suggested, that is interesting, as this is a putative migratory species.
My Flickr photos...
Certainly at sea. Over two days we have had about 10 of these guys. I would suggest not migrants though as southerly winds have carried them off the coast along with a host of other flies. Dipterist heaven.
Could be migrants as well. Southerly winds will presumably bring them up the north sea and then they would make landfall on the east coast. Perhaps a bit of both going on?
Lat/Lng: 53.2, 1.0
OS grid ref: TG0071