I think its a 1 CY Lesser Black-backed Gull, because there are some hints that are better for this than for Herring:
-pattern of greater coverts
-small head with slender bill
-I can imagine seeing a dark head with the right dark area surrounding the eye and ear-coverts that is better for LBB
-the tertials have a irregular border to the dark centers. Thats within variation for LBB, they appear not clearly notched to me. I know a 100% ID-book LBB should have narrower pale edges to tertilas, but thats within variation.
-I find it hard to asses the exact darkness of this bird, so I wouldnt argue that this bird is to pale for a LBB.
But there are two caveats:
I am no Gull Expert and normally learn from others here (Thank you to all, so helpfull!), so I hope to be corrected and learn!
I know there is much individual variation in Herring Gull, I couldnt for example ID this bird:
http://www.gull-research.org/hg/hg1cy/h131304aug.htm