Black-headed gull with bling

I had a drive around town today looking for waxwings and anything else interesting. I dropped in to the Johnston Gardens to see what ducks were around, but the water was half-frozen, and there were only a couple of mallards, along with a sizeable group of black-headed gulls. They were all in winter plumage (see Extra) apart from this one, and I only noticed after I downloaded the image that it has a yellow leg ring. I've reported the details, and hope to learn where it was ringed in due course.