I tried changing the code in the block_site_main_menu.php as you suggested but no change. Whne the page first appears, i.e. nobody logged in, the participants list can be viewed. If you log in as a guest, it can also be viewed. I will have to remove the block altogether if I can't find a way around this as I cannot have all my students details viewable by the public. And if I remove it then this means there is no way for students to see the full list of all other studentsd as they are all in groups on the courses themselves. Any other ideas?
You can see what I mean here: