Definitely Common Gulls, and a good description which makes it easy to verify what they were.
Although they're called "Common Gulls" they're not the most common gull at all, however they do tend to gather in reasonable numbers in town parks and on school playing fields in Manchester in the winter, along with the Black-Headed.
Hi, We have just done the RSPB Big School Birdwatch and we were counting the number of black-headed gulls which visit the playground after the children's morning break to feed on dropped crisps etc. We noticed that a few of the flock of approximately 20 birds had yellow/green legs not red. I presumed that they were perhaps juvenile birds at first but then I got to wondering if they were a different species. They seemed to be slightly larger than the black-headed gulls. Any thoughts? John