I authorized (from within gnome) to autoupdate my kernel. Waited for it to complete then rebooted as instructed.

Now the system won't boot ... (grub error 22: No such partition)

My /etc/sources.list is using a primary debian mirror: au.debian.org

I used the netinst CD to open a shell for rescue and checked the / partition - it is still formatted and files are readable.

I checked hdb1/boot and it contained vmlinuz-2.6.18-4-486 and initrd.img-2.6.18-4-486 which are referenced in hdb1/boot/grub/menu.lst

Code:
title		Debian GNU/Linux, kernel 2.6.18-4-486
root		(hd1,0)
kernel		/boot/vmlinuz-2.6.18-4-486 root=/dev/hdb1 ro
initrd		/boot/initrd.img-2.6.18-4-486
savedefault
n.b. - my MBR is on hdb - it points to hdb1
hda is not formatted

What could I do to get my system up and running again?

... any advice much appreciated - thanks