So I managed to screw up something and end up with confusingness in my boot options and they all don't work and just land me back in the
BIOSUEFI config.
I was following the waffley Beginners' guide and I think the issue is that I missed mounting /mnt/boot the first time round, which caused the "bootctl install" step to fail, and when I redid that I should have also redone the fstab command but I didn't so even though the installation completed it wasn't there after reboot.
Oh... or, it might also be because I didn't read the line "When successful, create a boot entry as described in systemd-boot#Configuration (replacing $esp with /boot), or adapt the examples in /usr/share/systemd/bootctl/." and haven't done that bit either. Argh! I swear I used a better guide last time round. :/
Anyway, both those should be fixable just by re-booting from USB and running the relevant commands, yes?
Or is there any reason I might want to re-format the disk and start the whole install again?