OK, I think I've sorted this now. So for the benefit of others:
I wasn't really going about this the right way. Some files like shared libraries, /proc etc. cannot be restored to a running system. So, you need to boot your machine using a rescue disk. There is a boot disk how to athttp://www.ibiblio.org/pub/Linux/doc...Bootdisk-HOWTO
. The rescue floppy needs to have the modules for your tape drive, and you'll need tools like tar, mkswap, mke2fs etc. You'll need the libraries required for these commands too (or use statically linked versions). Also you need to be sure your tar backup is relative (leading member names removed, tar does this by default). Boot up using the boot disk, and then recreare your partitions, and swap partitions. Format the file systems, mount the new root under /mnt, and also mount the others filesystems. Restore your backup:
tar xvf /dev/st0 -C /mnt
Reboot and run lilo to restore the MBR.
If all this sound too daunting, like it did to me then help is at hand. There is a tool http://mkcdrec.ota.be/
called mkcdrec. Using this you can create a bootable iso image and burn it to CD. Also you can backup your files. By booting from the CD you can run a script dynamically created by mkcdrec that will resore your system. Definitely worth it.