I've searched the forum and couldn't find any information so thought I would post this to help others.
The remote key fob contains one battery, not two like most other remotes which may explain its' short life.
The battery in mine was a Panasonic, but the important bit is the size which is 2032 (you can forget the bit such as CR or B before the numbers).
I found the following website that sells them for £2.49 each delivered, may not be the cheapest on the whole internet but it's easy to navigate and simple to use.
http://www.speeding.co.uk/acatalog/Batteries.html
Replacement is straight forward as long as you have a small flat blade screwdriver to unlock the screw in the key recess.