Dartford Marshes, 5th December 2010, Andy Lawson.
Many birds at this age look very similar to 1st winter Yellow-legged Gulls. In flight, however, this bird showed solidly dark inner primaries and a broader more solidly dark tailband. Not all Lesser Black-backeds at this age are this small and some Yellow Legged Gulls are small! I struggle to id many of these.