ps - going back to post#1 and the misbehaving book:
looking at it again., it styled every body text paragraph with
<p class="chapter">...
i think this triggered calibre's chapter detection so the conversion became a book with about 5000 chapters ( the conversion ran for ages) and then sigil choked on the result and could not open it. is there an upper limit on how many chapters ( Xhtml files) sigil can handle within one book ?
just a theory
but yes, re-converting with the xpath chapter detection defaults removed in calibre, sigil is happy to open the output, and the conversion went much faster. I should have spotted that at the time, as it's not the first book I've edited where having a class called "chapter", not being used on a 1 per chapter basis, has caused conversion problems. in past encouters though, the "chapte"r style has been used on the chapter header, so the only side effect was to put headers onto their own "page", with the associated body text on the next "page"