I eventually got SJ Content Category working even though the documentation bears little resemblance to that shown by clicking on the link in the Joomla Extensions page.
I am also very disappointed that the resulting layout also bears little resemblance to that shown via the demo link, again, on the Joomla Extensions page.
The result is that although I've ended up with roughly what I wanted (links to the sub-category pages via images), the layout is not at all what I was expecting - as indicated by Theme 2 on the demo page linked from the Joomla Extensions.