Play seem to be selling the Drop N Charge range by Exspect for substantially cheaper than I have seen elsewhere.
These sell for around £30 in Tesco for the charger unit and 1 battery pack then around £10 per additional battery pack.
This unit charges both Wii and Xbox 360 compatible packs.
The Exspect Wii Drop N Charge Kit includes a charging pad and a Wii Remote battery allowing you to fully charge your Remote without removing the silicone skin/Wii MotionPlus or having to dock the device!
* The pad holds up to two separate batteries which can be charged simultaneously or separately
* Charge an Exspect compatible battery device on the pad by simply placing within the grey box
* The LED indicators will illustrate the charge status of the relevant device by either a red/green glow