Hello,
a difficult picture, at least for me. As you have recieved no answer yet, maybe try it in the Bird Identification Q&A section?
I write my thinkings and hope like you for feedback from the experts, so I can learn, too:
Shade of mantle in relation to BHG in one picture seems better for a Lesser-black-backed Gull.
If you look hard enough, I see a long, parallel-edged bill, but rounded head and position of eye is not good for an ID-book Caspian Gull. But I must admit, when I would see this gull in NE-Germany I would see an realtive unmarked head for an immature LBBG (but still within variation with an brownish wash around eye) and think of the Gull-Mix that is prevalent there, and leave this bird unidentified.
I think your bird would be very advanced for a 2cy Gull with upper tertials adult-type and very advanced plumage, but large white tips to primaries may point to a 4cy bird, according to gullresearch?
Conclusion? Gull-experts, please help!