My grub menu finally got scrollbars so I decided to look into how to remove old linux kernels.
The easiest instructions I found are here.
I checked which kernels are installed in
$ ls /boot/vmlinuz* /boot/vmlinuz-2.6.32-29-generic /boot/vmlinuz-2.6.32-30-generic
And then removed everything apart from current and one version back.
sudo apt-get remove - --purge 2.6.32-28-*