I did think this was a black headed gull, but noticed there was no black on the tail or anywhere. Any help please...
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If you zoom in on the bird you can see the black on the primaries (not easy to see against the mud). This is the black that you normally see at the 'tail end' of a Black-headed Gull with its wings folded - there is no black actually on the tail of adults.
Even without seeing this it can still be identified as an adult Black-headed Gull, in winter plumage, by the black spot well behind the eye, and the relatively thin red beak with black tip.
Lat/Lng: 50.1, -5.4
OS grid ref: SW5835