Ok all, I've done some digging around it Lightdm is trying to start before UDEV reports the graphics devices as being ready. Please see #969489
By adding 'and stopped udevtrigger to lightdm.conf has fixed the issue for me.
start on ((filesystem
and runlevel [!06]
and started dbus
#Added the following line
and stopped udevtrigger
and (drm-device-added card0 PRIMARY_DEVICE_FOR_DISPLAY=1
or stopped udev-fallback-graphics))
or runlevel PREVLEVEL=S)
There is opinion that UDEV needs the full path to 'card0' but I suggest that for those with more than one graphics card (eg. Optimus) that a race condition could happen if your nVidia appears first. I don't know yet if this is possible though, someone with a better understanding than me of UDEV would have to comment on that.
Either way, I'm inclined to mark this ticket as a duplicate of #969489 as it is an older ticket with more substance. This ticket has a lot more heat so can I suggest that you all head over to the other ticket and mark it as affecting you?