Interested to hear from others more knowledgeable on details of 'mirrors' etc (ie pattern of white spots on the primaries basically), but I'd have thought pinkish legs and pale underwing, as well as what we can see and judge of upperpart colour and the clean white head suggest the adult is a Yellow-legged Gull Larus cachinnans of the form mongolicus, recently designated by the British Ornithologists Union as American Herring Gull Larus smithsonianus mongolicus.
Certainly interested to hear about the first winter bird.
Mike Turnbull
