[philja]

I've now had a chance to review some of the info referenced above. A promising lead looked to be Doitsu's reference to the Apple Books Asset Guide and I checked the link for which he included an excerpt. Unfortunately, that Guide is for Epub3 but it does provide a link for the Epub2 earlier version.

As that Guide says in its Intro:

Note: The 5.1 and later versions of the Apple Books Asset Guide cover EPUB 3 and Multi-Touch books only. For information on the older EPUB 2 format, see our documentation archive. Use this link to learn more: https://itunesconnect.apple.com/WebO...b2_asset_guide. You can also refer to http://web.archive.org/web/201904272...f.org/epub/201 for EPUB 2 information.

Sadly, when I clicked to go to the Epub2 version of their assets guide, my Apple ID login came back with a note that "Your Apple ID isn't enabled for iTunes Connect - Learn More". I clicked the 'learn more' link to see what was required but it all seems to be asking me what books or other material I want to distribute so I couldn't pass the first step.

I further noted the possibility :

Maybe Apple uses a similar method for NCX TOCs and their validator doesn't like the order of toc entries in the OP's book because they don't match the spine order.

and so, I have made sure that the Spine listing is identical to the order of the toc.ncx navMap listing just in case that has some role to play.

I have checked and rechecked that there are no spurious toc id's scattered in the body contents files of the book. Epubcheck gives a clean result so I'm going to have another go at getting D2D to pass a new version to Apple.