I've recovered the machine using a 'failsafe' logon, sufficiently that I can take a backup off the data disks. When I've got the data I need, I'll flatten the server and start again.
Thanks to MB and MG23 - and the others before - for your support.
I still have no idea how it is filling several hundred gigabytes of disk in a few weeks. I'm sitting behind a Netgear firewall and the standard SuSe protections are running.
Thank you for that hint bnt - I'll certainly install baobab - it will help me to keep an eye on the disk consumption.