Anemic Oak, Valloric,
I unzipped the original calibre epub and also the sigil epub, and found there were many differences. For example, there is a file toc.ncx for the calibre epub but no such file for the equivalent from sigil. In the calibre tox.ncx there is no h1 tag associated with the chapter titles (Chapter One, Chapter Two...) that are the basis for the toc headings. In the original html file that was used to make the prc file, the chapters do not have an h1 tag.
I'm a beginner in such code writing, but followed your suggestions - used code view in sigil, cut the entire file and pasted into Notepad++ where I added the <h1> & </h1> tags for each new chapter using Notepad++ search, then cut and pasted back into sigil, saved the modified file, and tried it in my PC Adobe DR - the toc was ok; also tried it in my Bebook and the catalogue key worked fine to give the toc.
Thanks for the help!