The Steam client informed me today that there were more recent Nvidia drivers for Ubuntu available and that I should upgrade for stability, etc. etc. It seemed a fairly innocuous change compared to the beta drivers I was using so I pressed the button and then restarted. Resulting in a failure to boot Unity and a chance to rediscover the joy of the command-line. I don’t know why Unity and the new drivers fail to mix to spectacularly however the simplest thing to do seemed to be to revert to the earlier drivers.
The problem with doing that is that I’ve only ever done it via the gui tools. This AskUbuntu answer told me about Jockey the software that underpins the proprietary driver contol. Running Jockey at the command-line was very, very slow but it did indeed allow me to select the earlier drivers and after a restart the GUI was booting again. Much easier than hand-editing an X config file.