Final update!
All sorted now! It was a permissions issue on my NAS. Because it worked OK from my other computer (running Slackware), I thought it would be OK on Libreelec. However, Libreelec seems to use a different UID for the user from Slackware, which is what caused my confusion. This was aggravated by me creating the "Recordings" sub-folder from within Slackware, which only gave the prime user read/write privileges. As the prime user under Libreelec uses a different UID, it couldn't write to that folder, although it could write to the folder above!
I set the path to the "Recordings" folder in TVHeadend.
All now works fine!
Thanks to Elonesna for pointing me toward the sample scripts for mounting NFS shares, which got me on the road to success!
--
Pete