If you 'not sure' or only think the gull you are looking at might be Great Black-backed Gull, then it's probably a Lesser? They are certainly uncommon in Shropshire but every now and then one drops in at a roost or elsewhere...
The first clue of GBBG jizz is - it's a brute and smaller gulls tend give them a wide berth.
If you are still unsure and there's a few Lessers nearby, the comparison in size is a no brainer decision! The mantle is always greyish black and clearly darker than andy of the Lesser races. The presence of black on the bill may indicate it's a fourth winter rather than full adult winter.
The barrel shaped body clearly seen here.
Off and away into the light...
