Thanks, I'll have a go at that later. I'm currently bogged down in a
completely unrelated grub issue on a different (wheezy) machine: I have
an ext4 filesystem which passes fsck fine, to which I can write, but
which grub2 cannot see.
The same grub rescue> prompt, ls can see the partitions, but grub2
cannot read the root one, says 'unknown filesystem'. An attempt to
reload after a chroot assures me that grub-probe cannot read it,
either. The grub-probe installed in the current Knoppix sees it as
'ext2', which may be as much as I can hope for, but wheezy's
grub-probe cannot see it at all, presumably ext4 is too recent for it.