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One or two adult-type feathers are beginning to appear on the back and upper wing coverts...grey rather than black, and so clearly pointing to Herring. The size compared to the Black-headeds is not as large as you'd see with a GBB, as well.
Individual gulls are never easy, though!
My Flickr photos...
Lat/Lng: 55.599, -2.435
OS grid ref: NT726340