Hi GMS,
To me, this is quite a tricky bird. It may well be a Caspian Gull, a less than typical one in some respects but variation occurs. However, given the existence of F1 and F2 hybrids involving Herring Gull genetic input on one hand, and variation in Yellow-legged Gull on the other, I don't know how confident I would be about this one purely from the evidence available.
To me, the tertial pattern seems to show a pale fringe to the outer webs back along a substantial portion of the feather's length. This pattern is something that I associate more with 1st cycle Yellow-legged or Lesser Black-backed Gulls, but, of course, no one single feature is diagnostic in these matters. The nature of the head pattern, with a heavily streaked nape contrasting with a whiter head, also seems interesting. Caspian Gull of this age class does, of course, have nape streaking, but the nature of the streaking here, almost being more in the form of solid blotching, reminds me more of Yellow-legged. The 2nd generation scapulars have rather large dark centres, which some Caspians can show, but this also would arguably be a better fit for a Yellow-legged or else be indicative of Herring Gull genetic input. I'd maybe feel easier about this as a Yellow-legged if it had some 2nd generation coverts or innermost tertials, but not all birds attain these during the partial post-juvenile moult, and, while the greater coverts look as though they may have been quite uniformly brown when fresh (i.e. not with the innermost coverts more notched/barred), this is difficult to assess.
Of course, living in Ireland, where Caspian Gull remains rare and not quite annual, I admit that I might look more critically at less than 'perfect' candidates than if I lived within their core range, so I wouldn't necessarily let my scepticism put you off too much!
Any stills showing the spread wing and/or underwing patterns better? And how pale did the underwing seem in life?
Regards,
Harry