Anyone know anything about gulls?

On the late morning constitutional detailed in the posting below, I snapped away merrily in the hope of getting a decent photo or two, and took this one of some black headed gulls near the Stone Quay. When I got home and downloaded it to the laptop, I spotted one of the gulls is different to the rest. The one in the centre has yellow legs and bill (all the rest have red) and it also sports a black edging to its tail which is absent from the Black Headed Gulls all around it.

Damned if I know what it is - any offers? And, yes, I do realise what a sad git this makes me.