© A. R. Dean

Generally, darker brown appearance compared with paler and greyer-brown livery of juvenile Herring Gull.
Inner primaries relatively dark, lacking obvious pale window of juv Herring Gull (though when wing fully spread, as here, inner webs are visible and still somewhat paler than outer).
Note also predominantly dark greater coverts, very broad tail-band with spotted base to outermost feathers and well-marked upper tail-coverts.