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But from the pictures here I am not sure I can safely agree. The mantle colour looks lighter than that of the BH Gull shown in the first picture (should be darker for Bonaparte's?). Bill does not look a little shorter than on the Black-headed Gull (although does appear to be black this can be hard to judge in photographs).
Underwing and leg colour not shown and these are useful feature too.
Were any field notes taken?
I don't think there is much doubt, looking at the head colour and the small bill, is there? Can't think what else it could be. Added to which, it was a known bird...you can find other pics of it on the internet.
My Flickr photos...
Lat/Lng: 54.9284, -1.3652
OS grid ref: NZ407594