What's impossible? Gull ID, that's what! Well, maybe not impossible, but always a challenge.
Here's a few pics from Stubber's Green today, which may or may not illustrate this point.
Yellow-legged gull - note the yellow legs
Yellow-legged gull - note the absence of any legs!
Common gull - not actually all that common!
Great black-backed gull - okay, that one's fairly easy
Caspian gull? No. Had us excited for a bit though!
Here's another picture, taken at the same site last year, of a probable Caspian gull.
Was it? Who knows?
I love gulls, me!