I got it working by entering the two top level DNS servers that my router uses rather than allowing TrueNAS to accept the DNS server provided my DHCP (which routes through my Pi-hole).
Plex is installed and the libraries are working perfectly. I know it’s basic stuff but it feels like a momentous moment after all the time I’ve spent!
Ir’s also awesome to see my TrueNAS dashboard come alive with activity. If I can get to grips with this OS it’s going to be brilliant.
Thanks so much for all of your responses (Henry and Sean) and for setting me on the right track to troubleshoot this. I would not have worked it out without your help.
Many Thanks,
Keir