Unexpectedly, one answer seems to have come from elsewhere.

To enable file sharing with this ancient RISCOS machine I had to enable
Samba v1 in Win 10. And now all the Win 10 machines are talking to one
another too. Bound to be some security or whatever issue, though.

--
*Never slap a man who's chewing tobacco *

Dave Plowman London SW
To e-mail, change noise into sound.