Once you establish that the black-headed gull would be more appropriately called the chocolate-faced gull it leaves the question "What is this gull with a really black head that you see on the coast of Dorset, mainly around Poole Harbour and, in winter, at Radipole?" This is the Mediterranean gull and it generally inhabits the coasts of Europe, especially the around Mediterranean and the Iberian peninsular and, in recent times, the southern coast of Britain. It was once only rarely seen here. I started bird watching back in the 1970's and when one arrived near us at Titchfield Haven in Hampshire it caused quite a stir and ended up being featured on the front of a book about the Birds of Hampshire. The Dorset Bird Report (2007) describes it as a rare breeding species in Poole Harbour and a scarce wintering species, especially around Weymouth. This group I photographed were nesting on the lagoon on Brownsea Island.
Like the black-head gull it loses that black head plumage in winter and it becomes a bit more difficult to tell the two species apart. The 'Med' does not have the same vivid red bill and legs of the black-headed gull.