Thanks to Dave, Richard and Paul for the comments about the white gull. It was with half a dozen herring gulls when I first saw it (they all left when the black-headed gulls arrived), and it stood out because of its colour. There was nothing about it, except perhaps the size, that suggested herring gull to me, but probably the bright white affected my perception of its proportions - the bill looked smaller, etc. But as I'm unfamiliar with the subtleties of IDing white-winged gulls, I couldn't argue with your ID of a leucistic herring gull.
Annie