unidentified Bird at Cliffe Pools

can anyone please help me identify a bird I saw sat eve at Cliffe Pools please I would say the bird was the size of a small goose, was white, but not as bright as an egret, with some black markings. it was swimming and had its rear in the air as it fed I guess, and then walked on the sand bank like a wader, unfortunately the light was poor and they were along way away. They were in the pool where the Avocets and other waders were on the sand bar. I cannot find anything like them in my RSPB bird book, help please. I also saw a couple of curlews on the river mud flats! Miles