[Phil_C]

Here's what the chapter code looks like from Book Designer when opened in Sigil:

  <p><span id="title"><a id="BdToc_1"></a></span></p>

  <div class="sgc-3">
    <span class="sgc-6" id="title">1 Technician</span>
  </div>

When the same HTM file is dragged into Calibre, some changes seem to get made during the process. This is the same chapter location;

  <h2 class="title"><a id="BdToc_1"></a></h2>

  <div class="sgc-3">
    <h2 class="title sgc-6">1 Technician</h2>
  </div>

It now contains h2 tags. It looks like something more still has to be done with this before a true TOC can be generated in either Sigil or Calibre. I'll have to fiddle with this to see if I can get it to work.