The bird was just resting amongst the Herring and Ring-billeds.
And the money shot...
After that it went back to sleep. You can get a hint of some of the other vestiges of winter dark head markings, though again it's pretty distant.
Hybrids always have to be considered with gulls. I don't have gull references available just now so I'm not going to go into them in great detail, but the mantle color, eye color, head markings, tertial skirt, leg color, and wing pattern all seem very average for Slaty-backed. Various dark-mantled hybrids including GBBGxHERG, GBBGxGLGU, and whatever Gull-nasty is have been featured on various posts here, just click on the "dark-mantled hybrid" tag on the right hand column under labels.