Fixed it - seems there was a bug with Windows 10 - I uninstalled SMB V1.0…
I just copied a working entry from another RPI (dietpi) for the usb1 share and pasted it into the ‘broken’ one to ensure I had a working entry for the drive as follows:
cat /etc/samba/smb.conf
…
[usb1]
comment = Local Storage Drive
path = /mnt/usb1
browseable = yes
writeable = yes
create mask = 0777
directoy mask = 0777
only guest = no
guest ok = no
public = no
read only = no
valid users = dietpi
force user = dietpi
force group = dietpi
case sensitive = auto
I then had to re-mount on the other *nix machines (copied from the dietpi-drive manager to ensure they were standard mounts (as per dietpi v8.9.2 dietpi-drive_mount), so had this on remotes as:
//IP/usb1 /mnt/PiName cifs username=dietpi,password=XXXXXXXXX,iocharset=utf8,uid=localuid,gid=localgid,file_mode=0770,dir_mode=0770,vers=3.1.1,nofail,noauto,x-systemd.automount
Whats really strange is that I didn’t even touch the Ubuntu mount entry, yet it now decided to work:
//IP/usb1 /mnt/Pi37 cifs user,rw,_netdev,credentials=/XXXXX/CREDENTIALSFILE,comment=systemd.automount,exec,uid=admin,gid=admin 0 0
And now Windows access also works…very weird intermittent behaviour
@Joulinar thanks for your quick assistance 
I’m not sure what happened nor what really fixed it.