Dark legs, white back.
No interactions present.
Tail fork is quite long, bill rather fine and short. I'd go with Common or Arctic personally. Leaning to Arctic as the dark tipping to the primaries is neat and confined to the feather ends, not diffusing into the hand and it's got a rather dusky belly. Perhaps a first-summer bird, but I'm getting badly out of practice at terns these days.
Flickr pics: http://www.flickr.com/photos/72243477@N00/
There were plenty of adult Common and Arctic Terns around so I suspect that you are right.
Lat/Lng: 57.6, -3.1
OS grid ref: NJ3465