So after turning on Authentication in Zoneminder, a bunch of monitors disappeared from the listing of monitors - it only showed the first 5 monitors. I was also unable to add a new monitor as the button was greyed out.

I'd even made sure the user was allowed access to the cameras.

However I managed to solve the problem, as it seems that for some reason it defaulted to only showing Group 1. So by simply clicking in the top left on "group1 (5 Monitors)" then changing it to display All, the cameras appeared again.

Just thought someone else might encounter the issue, as after extensive Google searching I found nothing remotely related to this!