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The black markings on the head suggest Black- headed rather than Common Gull
It's also got a red bill, rather than pale greeny-yellow as Common has. If you look at the leading edge of the wing, the white flash stretches along most of the first feather, whereas Common has that area grey with a black tip at the wing tip and just a white spot in that black tip.
My Flickr photos...
Thanks both - I know almost nothing about gulls and was identifying using a bad photo from elsewhere online!
Lat/Lng: 52.2193, 0.153
OS grid ref: TL471600