Gull... Help!

Photographed an unusual gull today. It was standing right next to an adult Yellow-legged Gull (as if that isn't remarkable enough!)

First the photos:

Mantle colour was the same as the Yellow-legged Gull. But everything else was different:

Bill was notably thinner without any obvious markings (almost Mew Gull like). It was duller along the inner half.

Eye was dark!

Leg colour was definitely more yellowish than the YLGU. In fact I would say that the YLGU legs were more orange in colour.

Legs were short, couldn't see above the tarsus really.

Head streaking was slightly more dense on top of the head, but was present along the neck too. It was a smudgy streaking, not very well defined.

Body was noticeably slimmer and smaller than the YLGU and HERGs. But seemed larger than a RBGU (although none of those were around for a direct comparison).

From the in flight shots it appears that both p9 and p10 are missing/growing in - so it is well advanced in its primary molt for an adult.

First impressions were of a Mew Gull/Common Gull type bird. But mantle was too dark, bird seemed too big, and there should be markings on the bill...

Thought about California Gull, but general shape doesn't seem right... but I have almost no experience with this species.

Also quickly dismissed Kamchatka Gull, mostly on probability and the fact that it wasn't on my radar. But one person has now suggested that that's what it could be... and whoa, some online photos of Kamchatka Gull look similar in some respects.

*Edit*

New photo shows the general size in comparison to surrounding Herring Gulls along with a Yellow-legged Gull to the right.

What hybrids come to mind for you?

Any other thoughts?

Never a dull day!

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Race kamtschatschensis breeds in eastern Siberia and, as the name indicates, Kamtschatcha. In winter it moves southwards along the coast, wintering in Japan, the Korea's and China. It is the largest race, almost as large as Herring Gull. Adult kamtschatschensis is slightly darker than canus and brachyrhynchus, but still paler than the central Russian race heinei
^from: http://gull-research.org/canus/canus5cyjan.html