This bird was a bit of a challenge. 5 years ago, I may have passed it off as a rather odd Greater Black-backed Gull.
Its appearance is ambiguous. The shape and structure of the whole bird, head, bill and legs are typical Caspian Gull. The bill pattern and leg colour are also more typical of Caspian Gull than Yellow-legged Gull. The lack of any 3rd generation scapulars is also more supportive of Caspian than Yellow-legged on this date. However, the underwing pattern and tail pattern are more typical of Yellow-legged Gull.
Caspian Gull can have dark (sometimes crescent-shaped) tips to otherwise white underwing coverts and axillaries, but this bird exhibits rather more than this, especially on the under primary coverts. Further, Caspian Gull normally has pale inner webs to all of the primaries including the outers - this bird seems to have very restricted pale inner webs from P7 to P10 - as in michahellis. Apparently, it is not unknown for Caspian Gull to exhibit these michahellis-like traits.
The tail-band (broader on the inners than the outers) is more frequent in michahellis than it is in cachinnans.
I had originally identified this bird as a rather odd, Caspian-like michahellis (which do occur), then Peter Adriaens wrote to say he considered it more likely a Caspian Gull, Pierre-André Crochet considered it not to be a michahellis. Requests for further opinions produced very helpful comments from Richard Millington and Richard Patient, both of whom believe it to be a Caspian Gull.
I have no idea what proportion of pure Caspian Gulls would have an underwing like this, nor whether this is a trait in the pure cachinnans gene-pool or whether it derived from some past hybridisation.
The 4th picture is the same bird taken on 22nd May at much closer range.
[The bird on the left of the 2nd picture is a 3CY michahellis.]