Fn key results in backslashes rather than brightness, etc

I recently upgraded from Ubuntu 14.04 to 14.10. On 14.04, the Fn key controlled bits and pieces such as mute, brightness, volume, lock touchpad, etc. all worked fine.

After the upgrade, they don't. Pressing the Fn key post-upgrade results in a backslash.

I've tried all the various options suggested elsewhere online to change grub's config:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux"

Suffice to say, none solve the issue.

I've since wiped Ubuntu and replaced it with Linux Mint 17.1, which I believe is based on Ubuntu 14.04, the version in which everything worked.

This leads me to believe there is some sort of setup/configuration setting which was wiped by the upgrade and is the default in Mint and in Fedora too, of which I tried a live CD.

Hardware Info: $ sudo dmidecode | grep -A3 '^System Information':

System Information
  Manufacturer: Acer
  Product Name: Aspire V3-771
  Version: V2.08

At a bit of a loss with this.