There are number of features clearly visible in these fabulous shots that don't ring true with identification as Yellow-legged Gull - or indeed Herring or Lesser Black-backed Gulls. The latter two species can easily be ruled out by the overall paleness of plumage, clean tail and rump pattern, all dark tertials and relatively advanced state of moult and wear. Structurally, the bill is relatively long, thin and lacks a significant gonydeal angle while in the lower image, the bird appears quite long-necked. The middle two images betray the bird's long and lanky legs - all of these structural qualities are suggestive of Caspian Gull. Another clincher is the stunningly white underwing portrayed in the second shot - only cachinnans would show such unmarked axillaries and underwing coverts, particularly at this young age - the body and head are also already very pale. Further good indicators include the greater covert pattern and the also the markings on the moulted scapulars - both visible in the lower image.
Despite British birders scrutinizing gulls more closely than ever, juvenile Caspian Gulls remain a genuinely rare sight in Britain with no more than a handful of records annually. There is only a small window in which they may occur on our shores: from the very end of July through to early/mid September, when birds' moult generally becomes well advanced towards first-winter plumage. Given this rarity and gulls' tendence to favour locations where, more often than not, they give distant views, Mick has done exceptionally well to capture a magnificent and highly instructive series of shots that illustrate this much sought-after age class superbly. And what a beautiful, elegant gull to to boot! These surely must be some of the best ever shots taken of Caspian Gull in the UK?!
You can find plenty more of Mick's Caspian Gull shots on this post on his blog, 'Birding the Day Away'.