My thoughts turned to Kumlien's Gull, but to clinch that I would need to photograph the spread upperwing. I didn't manage that on Tuesday, but with the help of a young Herring Gull today I managed to see get a glimpse (see the video). Initially it looked as though it had dark outer webs on the outer primaries, but further review of the video showed that it doesn't have the right pattern for a Kumlien's, so an interesting Iceland Gull it is in my opinion.
Juvenile Iceland Gull, Shawell, December 16th 2017
The Video is Best Watched at HD Quality.