My sons unity interface stopped working on 12.04 LTS, he is young and is prone to not shutting it down properly, I tried reinstalling gdm via this advice which described the problem

However apart from one boot up correctly which failed after I set it to autologin its got a bit worse. I can see the command line OK, so its not damaged and it boots up ok via a USB Ubuntu stick and to add insult to injury its dual boot and Vista is fine, now when I try and manually start lightdm I get

(Hardware RNG device inode not found.

"/etc/rc2.d/S21_rng-tools: Cannot find a hardware RNG device to use")

Any ideas?, I tried re installing the rng generator.