I'm afraid I spoke too soon when I said that my hidden users issue had been resolved in this post. (I am working in Moodle 2.3.3.)
The problem appeared to have been resolved as the users I had wanted to hide were hidden in the particular course I was working with at that time. I have since been to other courses and discovered that they are still visible participants in those courses. The course in which they are hidden is no different from any of the other courses and has no special settings. I had also created a new course and they are visible participants there too, despite the course:view setting being enabled in their role. It doesn't seem to matter if the course itself is old or new.
I have also tried changing their archetype from student to manager but this has also not helped. The situation remains the same.
I have tried assigning them as managers and they are then invisible to other participants, but they are then not able to see any courses in the navigation tree - we are using the sitenavigation plugin instead of the core navigation. This is, of course, a separate issue that I will post on that plugin's thread.
Why would it be showing them as visible participants in one course and invisible pariticipants in another? All of them are only assigned to one role in the system. Does anyone know what the problem might be and whether there is any fix for this? Any help would be appreciated.