Sometimes you don’t quite get long enough! On the Gull Masterclass Day of 18th Jan, I picked up the bird featured in the video below. It looked interestingly Caspian-like. Bit heat hazy, so got group on to it as I was jiggling around with video and it flew off. Not enough time to see all I wanted to on it, but certainly looks good candidate.
Sorry- quality is awful but perhaps by hitting the pause button a couple of times you can get a feel for its appearance. Extensive whitish underwing as it takes off.