Mystery bird I saw.

Hi, I have just signed up to Birdwatch, so I dont really know how to do anything on here yet. But a couple of months ago I went to RSPB Elmley Marshes, and took some photos. There is one photo I took and the bird in it is really strange, I have no idea what it is, but if anyone has any idea what it is, please tell me, I will try and describe it now. It looks like a gull, its body is white, but its eye and beak area has a black band around it. The top of the head is white.
I have checked all my books, but cannot find anything like it. I will try and upload a picture soon so you can see what it looks like. I am only 12, so sorry if my describtion is not clear, I tried to explain as best as I could .Thank you.

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