Have a look at the command fw_setenv
Thanks, Tim_Taylor. The advice above said not to use fw_setenv if the fw_printenv didn't show a valid result, I've got a CRC error, and I think it may point to a larger issue.
Also - the permissions on the file /etc/fw_env.config are set to 0664. I get an error when I try to modify the file with nano, and chmod says:
chmod: /etc/fw_env.config: Read-only file system
My Libreelec is on a SD card (not NAND) if that is material?