Thanks, David, that's most helpful and perhaps encouraging.
The navigation problem my friend reports is that the when she selects a link from the ToC to another section she is taken to the second page of that section, not the first page of the section.
The text font size in the ebook is set in ems. Most of the headings and titles are set in percentage of em. I have set the font to sans-serif; could this be causing a problem? It certainly doesn't cause any problem with my Sony; I can's select any of the other six font option ands the text changes to whatever I've selected.
Do you know of any other forum than MobileRead which discusses Kobo problems where I could get help?