Black Headed Gull (Larus ridibundus)
Firstly, some have different plumage in winter than they do in summer and that is no truer than with the Black-headed Gull. In winter it has no black head at all, just a 'comma' behind its ear. In summer its not black-headed either, it has a chocolate brown face. Not the best of names for this bird!
In Dorset this is one of our two most common species of gulls, the other is the Herring Gull. They nest in Poole harbour, especially on Brownsea Island lagoon, and in winter they are all around the harbour, in Swanage Bay, around Weymouth, especially Radipole Lake, where its is common the see over 100 standing in puddles in the car park fully expecting all cars to deviate around them.
Like the Starling, the Black-headed Gull is a bird with attitude. It is aggressive and noisy and its harsh call is like nothing else, just a rasping shriek.
The problem with Black-backed Gulls is they gather together in quite large numbers and other, much rarer gulls, tend to move in with them. You have to virtually look at every individual in the crowd to see if there is a different species lodging there.
Stand in the car park at Radipole and check the legs and beaks. If They are red then you have Black-headed Gull; if it's not but the bird is the same sort of size, then you have some thing else that needs a closer look - Common Gull or Mediterranean Gull perhaps or even something much rarer. At least you can always pop in to the RSPB visitor centre and find out what it is you have found.