Thanks you for looking for the additional photos and for posting the ones you have. These all are clearly White-winged Terns - the black underwing coverts are absolutely conclusive on that ID.
I'm not entirely sure whether the bird in flight with grey upperwings is the same as the bird I originally asked about - there are certainly some similarities.
I'm now inclined to think that the bird I originally asked about is actually a White-winged Tern, rather than Black. Besides the fact that no further photos of Black have turned up, I think that the leg length fits better for White-winged (Black is very short-legged), and the bill is perhaps too strongly curved on the culmen. It is a shame that nothing shows the rump or underwing to help with ID.
I had always thought that these two species were relatively easy to separate in breeding plumage, so for me it is an interesting lesson that the ID of this individual is not straight-forward. I will need to pay more attention to variability in breeding White-winged Tern in future!
