Graham: NVIDIA's installer broke Ubuntu's nvidia packages and the --uninstall option doesn't solve the problem that certain symlinks were overwritten by files, etc.
This creates serious problems when performing a dist-upgrade or installing Ubuntu's nvidia packages.
In another words what you're experiencing was caused by the NVIDIA installer.
Graham: NVIDIA's installer broke Ubuntu's nvidia packages and the --uninstall option doesn't solve the problem that certain symlinks were overwritten by files, etc.
This creates serious problems when performing a dist-upgrade or installing Ubuntu's nvidia packages.
In another words what you're experiencing was caused by the NVIDIA installer.