I worked this out as I used apt-get dist-upgrade which showed dpkg doesn't like cross filesystem links. Once I moved this back I had 60odd meg (I think this was du summing incorrectly as I didn't lose 16M of space) and ran dist-upgrade again. I saw my rootfs drop to 13MB at its lowest. I would suggest that 47MB is about the minimum, depending on what apps need to be replaced in place.