After 22 hours now, still FUBAR.
Restarted the network: DNS, Switch, DHCP, Synology, 272.
No change, no improvement
Removed Media Server - Intermittent high CPU load reduced.
Finally worked out how to install an old Media Server (1.6.2)
This one works: Music restored (Corelli Op6 currently playing); suicide risk abated.
I've found other people have had the same problem and I was right: The issue is confined to single core devices so almost certainly a threading / race error problem. This is utter crap: this release simply cannot have been tested properly on single core devices; completely unfit for purpose. Absolutely unforgivable disregard for their customers.
If it had been the OS I'd have totally given up on Synology and defected to QNAP, advising everyone else to do the same.