After updating to the new Ubuntu kernel I found that my brightness setting would only stop at the minimum and maximum settings.
Although it worked by echoing the correct brightness level via terminal, this was a very long winded way of doing it.
A simpler way was to edit the GRUB boot loader by typing:
gksudo gedit /etc/default/grub
Then changing the line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi="