I originally had this bird down as a Caspian gull, this ID seemed the best fit on account of its very white underwing, the solid greater covert pattern and the ‘soft focus’ appearance of the replaced wing coverts and scapulars as opposed to the heavier marked and more contrasty remaining coverts of 2cy Yellow Legged Gulls at this time of year. It obviously also shows the features that are shared by both Yellow Leg and Caspian gulls of this age.
I’ve seen it since a couple of times and have had doubts as to whether it might just be a clean looking Yellow Legged Gull with a plain white axiliaries and white underwing.
The views I had originally were mostly from a position situated below the bird, fairly brief and in dying light. Here are the problems ; The overall Jizz (the main thing thats bugging me), The iris is paling (which you can just see in a couple of the photos below and the last time I saw it the iris was even paler, almost bright.) Also no mirror on p10 (growing), not all Caspian Gulls at this age show it but it is apparently very rare in Yellow Leg, the bill is probably within range of Cachinanns but more suited to Michaellis. All this means its going down as a Yellow Leg, possibly with Cachi influence. Comments welcome