Think this photo with side on view to bill can rule out Caspian and the bill not heavy or blunt ended enough for YLG, scapulars don’t look right for YLG either,should by now show quite clear ‘anchor’ markings to feather centres -For what it is worth i think it is a Herring gull , albeit with a cleaner white than usual head…a nice flight shot, or as Andy says, a standing view would be good to further ID …definitely a tricky bird!
Another photo…
Glamorgan Bird Club 
Think this photo with side on view to bill can rule out Caspian and the bill not heavy or blunt ended enough for YLG, scapulars don’t look right for YLG either,should by now show quite clear ‘anchor’ markings to feather centres -For what it is worth i think it is a Herring gull , albeit with a cleaner white than usual head…a nice flight shot, or as Andy says, a standing view would be good to further ID …definitely a tricky bird!
Above should have read mantle and upper scapulars(probably ruling out YLG and bird a 1st CY
Thanks Trevor, would have been better if I had been able to take more photos.