In answer to my own earlier question, running lilosetup from the USB key doesn't seem to work, though maybe that is because of the mess I have made down here. I've just spent a good while trying to boot into the (as far as I know) undamaged Arch installation, without success. From the USB's grub, I pressed e and edited to what look like sensible values, taken from Arch's /etc/default/grub:
root (hd0,6)
kernel /vmlinuz26 /root=/dev/sda6 ro
initrd /kernel26.img
but when I tried to boot using CTRL-x, grub rejected this and put me back in edit mode. Arch is on sda6 with separate /boot on sda7
Once I get inside it, my plan is to do grub-install, in the hope I can get everything back!
I'd be most grateful for any quick pointers - it is all getting rather frustrating
