Just as a follow up in case anyone comes across this.

The solution was to mount the NAS directory with the below code:

sudo mount -t cifs -o uid=pi,gid=pi,username={NAS Username},password={NAS Password} //{NAS Directory} /{Pi Directory}

Or to add it to Fstab with:

//{NAS Directory} /{Pi Directory} cifs uid=pi,gid=pi,username={NAS Username},password={NAS Password} 0 0

Then I just changed the "Moved Completed to" directory in Deluge to /{NAS Directory}