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The yellow legs, together with the dark wings, point to Lesser Black-backed. I could have gone a step further and gone for graellsii, the 'British' subspecies.
My Flickr photos...
Thanks for the ID Ian. This one really had me confused. I ruled out LB-b because, for some reason, I've had it in my head that they did not occur this far inland.
They are common inland at this time of year. On school fields local to me there are none through most of the year, but up to 100 can be seen together on late summer mornings.
I'll try to remember that for the future
Lat/Lng: 55.6014006, -2.4427233
OS grid ref: NT722342