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It was about the size of the dance fly Tachypeza nubila
I'd assumed it was quite large, for some reason! Perhaps something like one of the Sciaridae, then?
My Flickr photos...
My guess would be Sciaridae as well, but not certain of that - last time I thought I'd found an interesting sciarid it turned out to be family Cecidomyiidae!:
Entomologist and biological recorder
Thanks. Wondering if the fly I posted under "Late flies on ivy (cont.)" could also be Cecidomyiidae. Perhaps more will turn up.
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707