So. When I last reported on my progress, I had formatted my proect text in Pages, and exported it as an ePub file. Unfortunately, though the result was pretty, the Table of Contents was stripped out during the conversion process. And if I was going to the trouble of making it an ePub and not just a PDF file, I wanted my browsable Contents feature.
Eventually I found Sigil, a free WYSIWYG ePub editor. I stripped away all styling from my text and made clean HTML files, moved the files into Sigil, learned how to use Sigel, re-tagged and re-styled all my chapter headings and sub-headings. And...
It all worked.
I'm happy, and that Christmas present is all done!