Exe 'white' Black-headed Gull

The Black-headed Gull referred to in Dave's post below flew over Bowling Green Marsh yesterday evening and carried on down the Exe at 2030hrs. In flight I initially thought that it may be a Med Gull, but when I saw that the mantle and upperwing surfaces were white and not pale grey I concentrated on the other features in the short time I had. As Dave mentions this bird had a red bill, and dark eye, making it in my view leucistic rather than albino. Not sure about the chin shading though - perhaps it's not shaved lately? I saw a similar bird, but not so white as this individual, at Powderham some years ago. Other birders beware.........