OK guys- have a go at this
S
Being a relative novice to bird watching I can only hazard a wild guess . So I'll say some type of Tern which has black legs ! bet it's a rare one knowing your expertise and not seen very often in UK ....Caspian ?
The bird looked quite large in D's photo and I was lucky enough to see a Caspian Tern three years ago (around this time of year) at Leighton Moss with its huge carrot bill although in flight I had nothing to compare it with size wise as it flew in circles over the Causeway pool. I really hope it's a CT - please say it is lol Would love to see one again.