Hundreds of small (2mm) black flies stuck to a sign post.
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They look a bit like Bibionid flies, but 2mm is very small, so I am not sure.
My Flickr photos...
2mm was just a quick estimate of body length. Taking legs and folded wings into account, some could be 4mm. Still small though.
When I saw a similar thing a few years back I put them down as Scatopsidae - relatives of the Bibionidae. But I didn't have the enthusiasm to take specimens and key them out to prove it! ;)
Lat/Lng: 54.672, -5.758
OS grid ref: NW577372