This could be a pure cachinnans and even though one may argue that most of the features seen in this bird also fit argentatus, there are firm clues that at least one of the parents must have been a cachinnans. Even though the upperparts are rather pale; head relatively large and striated; and p10 lack white mirror, it would not surprise me if it still fits within the phenotypic variation found in true cachinnans. However, a hybrid or backcross is not easy to rule out and on current knowledge this may even seem like the most probable solution. See other comments on Gull Research.
Photoes were taken on February 2, 2013 at Vejers Strand in West Jutland.
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