Immediate reaction is Caspian, but theres Yellow-legged feel to the bill, a bit short for Caspian, and maybe to much curve to the bill tip, and strong gonys angle, although gonys matched by some Caspian (males) Tertials looks Caspian, without any notching and white tips not reaching tertial base, but could be mached by some Yellow-legged. Caspian with it´s long primary projection (usually a bit longer than YLG) isn´t seen here because of angle Without going to much in to plumage details I´l be tempted to call it a Caspian, with a possibility of it being a Yellow-legged! I suppose you don´t have some more images? Taken today at Simrishamn, where I have been many times watching gulls, I looked to see if there where any other images, but no. It´s a rare occasion not being able to pinpoint a Caspian, in such good images or a Yellow-legged for that matter. More images!
JanJ