Ann - I suggest Alan Caiger-Smith of Aldermaston Pottery.
mark - sure it is aldermaston
Mary - Certainly looks like it but doesn't ACS usually have a date symbol, too?
potz100 - It is Alan C-S and is the mark used at his Homer Street, London studio between 1956 -63. Hence lack of date mark which would only be used at parrallel Aldermaston.