Several other people came in (Ian - SBBO warden and Adam) who also thought it looked different. Ian has since come back and is happy with it being a Baltic Gull (fuscus).
Attached are some shots that show the features that lead to this conclusion. Unfortunately I didn't manage a clear shot that shows the upper side of the wings and mantle. The best I can offer in that department is where the bird was flapping whilst preening.
|Lesser black backed (left) and Baltic (right)|
They also stood close together on the emerging island and the size difference was clear to see.(no pictures though).
This is one of the clearest views I captured of the upper wing but the lack of contrast between the upper wing and primaries is clear.
A few more shots of it flapping around whilst preening:
Finally the best shot I managed of it in flight:
An interesting interlude to what was otherwise a quiet day.