[roger64]

Maybe a simpler solution (at least for ODTImport users) would be:

1. On your odt: create a dedicated chapter title for the toc and place it where you wish in the structure of your text files.

2. After conversion, using Sigil, you see it's, say, chapter73.xhtml. It contains only a h2 title (T.o.C) and a link to your stylesheet. You see also that the brand new toc.xhtml has been dropped at the beginning of your text files.

3. Copy and insert this h2 title (with its id) and the link to your stylesheet and the namespace in the toc.xhtml file.

4. Delete the original chapter73.xhtml and rename toc.xhtml (modified) as chapter73.xhtml.
Move it to its new place.

This way, you do not have to take care of the properties="nav" attribute.