Problem solved
Looks to my amateur eyes like an IP address conflict.
Nice try, but no IP address conflict. Yes, I do use DHCP but every machine that lives on the network gets a fixed IP address and I can see that this is working. The only IP addresses handed out from the pool are those for visitors' machines that I let on to the network.
I have however now solved the problem by renaming the Workgroup on the NAS, powering it down (controlled), renaming the workgroup to match on each computer, and then rebooting the NAS. Bingo. Solved - even if I still have no idea what triggered it. Between it working and not working I hadn't even done my regular PC housekeeping let alone install anything knowingly.
Anyway - Happy Camper.
Thanks for the help along the way, All.
XV