Tern ID needed

The bird in the photo below was seen at Rye Harbour LNR in East Sussex on Tuesday 22nd June 2010. Please click on the image for a larger view. (The second photo being a heavily-cropped version of the first.)



It was seen from the Denney Hide and stood out from the many Common Terns present.
Paul managed to get the photo above.

It tended to keep away from the rest of the terns and appeared to have a dark bill with maybe a trace of red at the base. The bill seemed to be slightly thinner than on Common Tern and the bird appeared slightly lighter-built than the rest of the Commons present.
To me, the legs appeared too long for it to have been an Arctic Tern (and I wouldn't have expected to see an Arctic along the south coast towards the end of June) and, although the photo does not really show it, as the bird was at distance and seen through a slight heat haze, the legs were a slightly-orangey red. Neither did it show the 'white cheeked' appearance of an Arctic Tern.
To me, the upperparts seemed a shade too dark for Roseate Tern but, as the bird always seemed to be facing us at the same angle, we never got a good side-on view to judge this colouration or its tail-length properly. Although from the cropped photo, I believe that I can make out the wing and tail lengths, but I may be wrong.



All the terns occasionally took off and landed again and this bird was easily picked out from the others at rest on each occasion. always landing in a position away from the other terns. Unfortunately, we did not, however, manage to pick it out in flight amongst the many Common Terns.
As a Roseate Tern has been seen on and off at Rye recently, including today (4th July), I am leaning towards this being a Roseate. Others have suggested it is just a Common Tern and one has suggested it is an Arctic Tern.

Your comments and opinion on this bird would be most welcomed. I don't mind at all if you shoot me down in flames over this bird, I would just like to know what it is.

Graham