Odd gull

Back in 1981, Toyah had a hit with a song 'It's a mystery.' I liked it at the time, perhaps not so convinced on looking again now. However it summed up how I felt looking at this gull. 
Is it a Common Gull? The bills looks too long and the head is not the right shape.
Is it a Herring Gull? It doesn't look mean enough and the leg colour is odd.
Is it a Ring-billed Gull? A bit rare although one was in the area a month ago. It doesn't look like others I've seen.
Is is a Caspian Gull? Rarer still here. Never seen one to compare although it looks quite close.
Any other offers? Advice gratefully received.