Steve, the Hide/Show for the TOC should only be an issue if you're launching the help from the Default Topic (your "Home" page) instead of from the Start Page (which contains RH's frame layout, and which displays the TOC or whichever navigation you chose; Index, TOC, etc.).
This doesn't have anything to do with the mini-TOC (which is actually an "In This Topic" feature, in spite of its name).
It appears that I need to do some reading up on how to use some of RoboHelp's features properly (time is a luxury as always and these are mainly inherited projects!).
The Show/Hide link is ONLY visible if you enter any text in the Caption field (thanks Peter). However, after reading your email, I generated the CHM of the project, as opposed to previewing the page. The Show/Hide link itself is then translated according to the language setting whereas in Preview it is in English!
I proposed to replace long lists of links to bookmarks in the same topic. As the bookmarks were all pinned against headings of the same level, a Mini TOC against a single level provided the perfect solution. In retrospect, asking an engineer to Show or Hide a simple list is perhaps one click too many; leaving out the caption will be better.