Found reduced in my local Asda Store.
Charge two Xbox 360 controllers simultaneously in just 2.5 hours with Energizer's Power & Play Charging System for 360. Fully compatible with Microsofts's Xbox 360 rechargeable battery (not included)!
Cheapest I have found it before is £20 + P&P - so £17 is a bargain!
More details here - http://store.pdp.com/p-753-energizer-power-play-for-360-charging-system.aspx