I have finally found a solution to the issue I first reported this issue almost a year ago here:
Plex/Synology NAS/VirginMedia Router and Sonos working only intermittently.
Eventually I rang Virgin Media and reported the Modem/Router (a SuperHub) as faulty. They replaced it with a SuperHub3 and - hey presto! It works reliably.
I’ve had this working for a month now - so I am reasonably confident that the fix is real.
Unfortunately the original thread is closed - so I can’t update it - so for the benefit of other I am posting this to a new thread (with the same tittle).
Hopefully this is helpful to others and avoids hours of frustration!