Thanks very much for the explanation..
Boy I do feel a little silly for asking the question after checking out the epub created by Sigil a little more. Actually by simply saving the text as epub (after checking with the TOC editor) would create an epub with an appropriate TOC. But this TOC was 'hidden' until I chose to view it via an option on my ebook reader.
I've often thought that TOC would be displayed as a list near the front of an ebook (such as those I purchased from fictionwise, and the RSS feeds I downloaded with Calibre).
Now I know better.
Big thanks to Sigil's creator. What a brilliant tool it is.