Official 360 play and charge kit for £10.99 instore at argos.
I also think this is the cheapast instore price for one of these!
Xbox 360 Official Play and Charge Kit - Black.
Xbox 360 console Play and charge kit.
•Recharge your Xbox 360 Wireless Controller while you play and kiss disposable batteries goodbye!
•Complete with a charging cable and rechargeable battery pack, the Xbox 360 Play and Charge Kit is all you need to juice up your Wireless Controller even in the midst of battle.