I think I might have worked it out.

I think that the moment that I'm not using an initrd at all, because my system doesn't have scsi or anything else that requires it. So those files in /boot really are just kernel images.

So I need to run mkinitrd to produce the initrd image in /boot, then add a new entry to /etc/lilo.conf, complete with an initrd line. Then run lilo.

About to try it...