@noddy
On my NAS4Free (N4F) I am using COPS (Calibre OPDS) and it works great, since any device browser on my LAN has access to the library.
Check out this
thread here in Mobileread and
COPS : Calibre OPDS (and HTML) PHP Server.
My Calibre library is on my desktop with a backup on the N4F, which is always available. I use
WinSCP to synchronize/backup my desktop Calibre to the NAS.
Cheers!