Well I invited comments and I'm pleased to say I had several: sadly, though, all anonymous. Two said Caspian Gull, three said 2CY Lesser Black-backed, so I'm happy to go with that.
This, however, raises an interesting question: why give your opinion if you're not prepared to endorse it with your name? Surely saying 'It's definitely a LBB' suggests a certain confidence in one's own abilities, so why fail to demonstrate the courage of one's convictions by remaining anonymous?
Just for fun: which (if any) of the birds below are Caspians?!