There are lots of gulls on Knutsford’s Moor at the moment, almost all black-headed gulls, as on the featured image. Then there is a single larger gull, which stands out from the rest, does its own thing, and is treated with some suspicion by them. It is not one of the gang.
Click to see as slideshow.
The lesser black-backed is typically half as big again as the black-headed variety, but significantly smaller again than the great black-backed gull. See also Cornell Lab info.