Well, this one’s resolved, so I’m putting up my fix in case anyone else has the same problem. If any of what follows is blindingly obvious, well, it wasn’t to me.
Seems like if the directory Volumio wants to use to mount the shared folder doesn’t exist it won’t create it, it’ll just spit you the error above. So, creating it manually then adding the NAS mount sorted my problem.
Get hold of PuTTY, SSH into Volumio using its IP on your network (user/pass root/volumio), and type the following.
chmod -R 7777 /mnt/NAS/SERVER
(Where SERVER, in both cases is the name you’re going to give your mount - can be anything at this stage. SERVER is fine )
Then go back to the Volumio web interface, add the share, making sure you’re giving it the right IP, Windows credentials, and giving the source name as whatever you put in for SERVER above. Remote dir should be whatever your music folder is shared as on the network. Worked straight off for me and sounds lovely through my AudioQuest Dragonfly. The only reason I’ll be switching back to RaspBMC at some stage is the Yatse Android app for controlling it, for which the voice recognition is stunning.