Caspian Gull X307, Fulham, London, 14 October 2017
I've seen X307 here on one occasion previously, in late July. As can be seen in the above, X307 is a quite demure and 'cute' bird with a dainty bill and lightweight structure. So, presumably a female. Curiously, there is only a tiny hint of the 'diagnostic' p10 mirror, which is just about detectable on the left wing. Perhaps not too surprising for a bird from Germany. Plumage otherwise looks pretty good.
The odd Yellow-legged Gull was seen over the weekend, including a couple of 1cy birds and this regular near-adult.