Thanks for your help so far. I'm not sure what's happening now, though.
If I now go in to the "Master override" (i.e. right click authors, Manage Authors), this now appears correct. Amy Stewart has a sort of Stewart, Amy.
However, if I right click on the title, and select the book, the sort order is still wrong, showing Amy Stewart as per the picture in my second post.
See images.
MasterSort shows there is no sort author of Amy Stewart
MasterSort2 shows Stewart, Amy.
Sort shows the author is still being sorted by Amy Stewart.
AuthorError shows what I see when I right click on the book.
Any ideas?
Edited to add:
If I change this author by right clicking on the book, changing the sort to Stewart, Amy, and clicking OK, this fixes it.
If I then use the "Master override", this doesn't change it back to Amy Stewart, it stays as Stewart, Amy.