Alright, it took quite a while however I managed to solve the issue. It wasn't just applying the beta, it was actually also updating the devices. It refused to acknowledge there was a standard update or a beta enrolment to undergo.
Resetting and restarting on it's own had no effect, however if you reset the device or restart while the device is plugged in, then you'll get the prompt. Occasionally one or two of my devices would work without requiring this after a few restarts, so I'm not sure what was happening there.
Anyway, thank you for the help.