Thanks but I was already authenticated. What seems to happen is that the UI programmers have disabled the menu if an admin has determined that auto-log-in should not be capable of being turned on. However if it is already on, this 'feature' prevents it from being turned off!
How, as an Admin, would I have disabled auto-log-in other than in the system prefs/accounts/log-in options? If I could find out, then I could allow it, then disable it!
Thanks