Common Gull (Larus canus)
I have a problem with identifying them in flight but when you get to see one like this one perched on my neighbours roof it is a bit easier. They are about the same size as the Black-headed Gull but bear very little resemblance to them and they are smaller than the Herring Gull and have a green bill and legs not yellow. They also lack the red tip to the bill that the Herring Gull has.
As with many gulls in Dorset, one of the best places to see them is Radipole Lake RSPB reserve and it is always worth looking amongst the Black-headed Gulls standing in the car park as you will often find a few Common Gull's in amongst them.