Confidence: I'm as sure as I can be.
Notes: An adult. Much larger than the Black-headed Gulls in the foreground and with a charcoal-grey back, darker than the Black-headeds too. At full whack you can also make out a reddish orbital ring. Looks like a 1st-winter Herring Gull behind (the other large one which is mottled brown) and a handful of Greylag Geese on the lake.