You need to run this 'dpkg-reconfigure lightdm' and choose 'lightdm', and you can show this by 'leafpad /etc/X11/default-display-manager' which should also show 'lightdm'. Then amend 'leafpad /etc/lightdm/lightdm.conf' to show ;- [LightDM]
start-default-seat=true
greeter-user=lightdm
minimum-display-number=0
minimum-vt=7
user-authority-in-system-dir=false
guest-account-script=guest-account
log-directory=/var/log/lightdm
run-directory=/var/run/lightdm
cache-directory=/var/cache/lightdm
xsessions-directory=/usr/share/xsessions
xgreeters-directory=/usr/share/xgreeters
[SeatDefaults]
#xserver-command=X
#xserver-layout=
#xserver-config=
xserver-allow-tcp=false
#xdmcp-manager=
#xdmcp-port=177
#xdmcp-key=
greeter-session=lightdm-greeter
greeter-hide-users=true
greeter-allow-guest=false
greeter-show-manual-login=true
user-session=default
allow-guest=false
guest-session=UNIMPLEMENTED
session-wrapper=/etc/X11/Xsession
#display-setup-script=
#greeter-setup-script=
#session-setup-script=
#session-cleanup-script=
autologin-guest=false
autologin-user=
autologin-user-timeout=0
#autologin-session=UNIMPLEMENTED
exit-on-failure=false
Save, and then reboot, and it should set you up with lightdm as your login client.
Good luck
Sharon.