I thank you all for your help with this.
Hitch... your experience with Smashwords is not very encouraging, but thanks for sharing some history.
theducks... thank you... and I know it sounds insane, but there are no files created by iTunes in the book. All folders are empty with the exception of Images which contains one cover image... and the Text folder which holds everything else. I've tried searching the computer with every possible connotation of iTunesMetadata.plist and nothing is being found. I give up. I'll have to rebuild the book and never set foot in iTunes again.
Gunnerp245... thanks for your suggestion. I've had a look at the 'modify epub' plug-in for calibre but I'm too stupid to understand how it works.
Again... many thanks to all.
Helen