Comment 22 for bug 1089195

As for Unattended Upgrades, you can configure it to remove unneeded packages automatically before a dependency problem or an inode problem occur:
https://help.ubuntu.com/community/Lubuntu/Documentation/RemoveOldKernels
I am not sure, if it works on 12.04, though, but I have used it successfully in 14.04 and 16.04 (using the old method).
As for purge-old-kernels script, it has some bugs. It does not purge common header packages (matching ^linux-headers-[0-9.]+-[^-]+$) for instance. I have been developing a more advanced script that can do some troubleshooting, as well. The following video showcases normal purging it can do; the --fix option is not covered by the video:
https://youtu.be/_UYrB6XBpr8