Clothilde. Which, for sure, wasn't this UK girl's name.
Cotty might have been one of those family nick-names that make no sense to anyone outside the family. In my mother's family, she had a sister named
Elsie, who was always called
Pat, even her husband called her that. Also a brother named
Edgar, always called TT.
Not a very intuitive nick-name for
Charlotte, I don't think.