We need to see another image of Martyn,s gull to to have a fair chance for a possible identification. From this image alone, itīs impossible to tell YLG from CG from argentatus HG.
Some structurally features can be seen - which might lead to a possible so and so gull.
I think itīs more the white-headed and dark upperparts tone that lead onn to a YLG/CG suggestion, in fact there is realy nothing in this image that doesnīt fit a Herring - structurally, except dark tone and white head - which would fit some argentatus.
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