No idea what it was but I had already done the following:
1) rebooted the router
2) rebooted all of my Sonos products (Arc Ultra, Sub Gen 4 and 2 x Era300),
3) uninstalled and reinstalled the Sonos app
4) tried using the app on an iPhone and Android phone.
None of the above worked, however, I then went into the Sonos app, settings, help centre, update network and ran through the procedure to update the network and now everything works as it is supposed to do, including allowing me to re-authorise Apple music.
Thanks @Airgetlam as your post sort of pointed me in the right direction with the "Network Refresh" comment