and there is an issue with DNS rebind, and after reading have now added
rebind-domain-ok=/plex.direct/
to the DNSmasq settings
The above was definitely required to avoid Plex server bombing out every 30 minutes. uPnP didn't make a difference.
So once again thanks however....
o The router will auto connect to SKY FIBRE on startup
o DHCP now renews successfully every hour, and then every 24 hours
o Plex now plays a movie all the way through without crashing
So hopefully everything will continue to work without problems!!