I decided to experiment with the new (to me) slackpkg way of keeping Slackware up to date. My home server was Slackware 12.2. I decided to see if slackpkg could get it up to Slackware 13.0. I tried it. It was a dismal failure.
Once I determined that I had rendered the server unusable, I installed Slackware 13.0 on it, configured it and got it all working again, and then used slackpkg to get it up to date. It worked fine.
So I have learnt an important lesson. Don't try and jump versions with slackpkg. I'll opt for the safer upgrade path in future.