No sooner had I posted then I found the cause – it’s my theme. Switched to another and it appears.
I suspect it’s to do with the fact that it’s not pointing to a physical page. Can I create a page as the root and use a shortcode to display the contents? When I’ve tried to change the root name to my page name it throws up an error.
I had the exact same issue. Switching to a basic theme didn’t work but deactivating all plugins did. However, after re-activating them all one-by-one the issue hasn’t re-appeared and the Forum options are still there.
If it helps anyone get to the bottom of this issue, when I first activated the plugin and tried to direct me to the welcome page I, instead, go a message along the line of “Insufficient permissions” which was odd. I’m guessing it was then a permissions issue that prevented the menu options from then appearing in admin.