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My first post. The longer I look at this the less clearer I am. It's obviously big (compare with marinus), very square headed with long robust bill. Any thoughts please?

I would go with a rather odd Herring Gull.

Looks like a big Male Tatus. I've caught male adult Herring Gull in southern Norway that on size and measurements would, if it was a GBBGull been a big female.

Thanks for the observations. There have been a few adult & first winter argentatus here in the last month so a maximum sized 2nd winter is a nice addition. At the other end of the scale there was a very small argenteus at the same site on Friday. I doubt it was more than half the mass of this bird.