Firstly i apologise for the rubbish images you are about to see, was pushing capabilities to the limt here!
Now, on to my unidentified tern sp. I was attracted by 2 common terns acting cautiously and alarm at a 3rd bird, as i looked this bird waas noticeably different, main features noted were; Uniform sooty grey all over, on underside, above and on the wings as well. The sooty grey appeared to come upto the base of the bill, which was all (dark) red and looked robust. this gave the bird a very notceable and distinct "white Cheeked Pattern" i did hear it call, very hard to describe though, rather like a kingfshers whistle pitch and with an "rek" sound to it, hope that makes sense. The tail seemed a little odd, as it appeared swallow like. also dark trailing edges to the wings
Hope you are able to help me come to a conclusive id,
Many thanks in advance.