I’ve had to change my iTunes account, finally embraced FamilySharing and totally different accounts, rather than a single iTunes account that we all shared.
As a result I need to re-register via TestFlight for Plex iOS Beta, sadly when I try to do this I’m getting an error that the Beta is full. TestFlight supports up-to 10,000 users, so I’m hoping this might just take a little bit of config change, or perhaps a more general purge of old/un-used accounts might be needed.