Right, I've had a hunt and it's NOT a battery issue.

"The wireless controller does not function.

When using the controller wirelessly, if the controller and the system are too far apart, you will not be able to operate the system effectively.
When using the wireless controller, you must pair it with the system and assign
it a controller number. With the system turned on (power indicator on the system
front is lit solid green), connect the system and the controller using the USB
cable, and then press the PS button on the controller.
Check the charge level of the wireless controller battery. When the controller is
used in wireless mode, it will not function if the battery does not have any charge
left. Charge the battery by connecting the controller to the system with the USB
cable.
Wireless controllers for the PS3™ system are not equipped with a vibration
function and will not vibrate when using software that support the vibration
function.
Reset the wireless controller by pushing the reset button on the controller rear with a sharp-tipped object such as a pen"

It seems it's quite common and by resetting it (Tiny reset button on rear of controller) is what's fixed it for most people...