unity - Ubuntu automatically logs in as gdm how do I stop this?

Not sure what I did but as of this morning ubuntu on my imac has been auto logging in as gdm. I've tried the following remedies

1. reinstall lightdm and ubuntu-desktop


2. delete auto-login and set auto-login-guest=false on /etc/gdm/custom.conf and /etc/lightdm/lightdm.conf


3. set up a new user with identical permissions to my old one and try and re-login with that.


4. upgrade to roaring-ringtail beta (I have nautilus 3.7.1 installed and I wondered if this might have had something to do with it)

Wierdly one of the effects is that when I open a terminal it opens and then disappears. I have to go to the tty terminal using Alt-Ctrl-f2 to make any changes. Not too sure what files I need to post to help resolve this, but would appreciate any help.

edit: the home directory for gdm is /var/lib/gdm (not sure if this helps)