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Apart from size (with nothing to compare to here) very dark back (again tricky with no comparisons - though it looks pretty dark) the only way I know to tell between the two is with the legs. Now it may be my imagination because it is far from clear but the legs look to have a pinkish hue which makes this Great Black-backed... (Larus marinus)
Feel free to look at my ongoing (hopefully improving) collection of pictures on Flickr:
It was noticeably big (sorry, not that helpful); it was bigger than the Herring Gulls nearby and definitely bigger than the Black-headed. In the full frame, the only size comparison is a distant windsurfer!
I'm glad others find separating these species difficult when in flight. My eye was taken in this shot by the shape of the bill(very deep)and the width of the white trailing edge to the wings (which I often find difficult to see on Lessers).
Hope this helps - a bit.
Alan Brampton (Benson)
Lat/Lng: 51.3647, 1.0263
OS grid ref: TR107671