It is private. After doing some digging it turned out that a cache plugin that was still installed but deactivated was causing the issue.
Its sort of sorted it out in a sense that I have 4 forums and 1 of them can now be viewed by participant users. I’ve no idea what could be causing only one of them to be made viewable and not the other 3.
Hi! It turns out that the forums do work but the Oh bother! No forums were found here!’should really say that people need to be registered/logged in to view the private forums as opposed to saying they don’t exist.