I picked this bird up initially in flight, where the "venetian blind" upper-side and almost completely white under-side to the primary tips made it a very obvious Thayer's Gull. It then landed, when it became a lot less obvious - I grabbed a camera and took the first 2 shots below (it is the bird on the right in both). A gigantic trawler then barged into the scene and I lost the bird. I gave up after about 20 minutes. Note the under primary pattern and the paler shade of grey relative to the adult Vega Gull.
On returning to England, I noticed that the next picture in the camera, which when I took it further along the quay I passed off as a Vega Gull, also has a whitish under-wing tip. Sharp-eyed Richard Millington asked why was it not a Thayer's Gull? If you compare it with the first 2 pictures, it's obviously the same bird! The 4th shot is an even better picture.
Apparently, Thayer's Gulls have become reasonably regular in Japan, and if you look at a globe, the distance between the breeding grounds and southern-most California is similar to the distance to Choshi - just a different direction.
Whereas this bird might have been conspicuous in Europe because of its dark eye, in Japan, many of the Vega Gulls are also dark-eyed.
Postscript: As a result of posting this bird, I have heard from Osao and Michiaki Ujihara that they had already found 4 adult Thayer's Gulls at Choshi this winter and this was the fifth! Not only that, this same bird, reckoned to be a large male, was photographed last winter at Choshi and so must be commuting between north-east Canada and Choshi. The pictures can be seen here and here.