Thanks again. I optioned my Calibre output to epub and sent that file to the epub validator, and got back a massive list of errors about a thousand lines long. (When in the past I've output to epub then converted back to mobi using the Kindle Previewer, it worked ok.)
I downloaded Sigil but it always crashed as I opened it.
And I've got someone else's cat hiding in my attic!
Thanks for all your efforts anyway,