NixOS is up and running.

I tried the manual way through the terminal, and that didn't quite work either, but at least it deleted the old partions, and then the interface installation completed.

The only error I got was the unfree software. That step never showed me the dialogue box, just the error, bit I skipped over it.

I managed to make some changes to the config file, tested one rebuild and rollback, and now I can see the two generations listed. Cool. I like it.