Just bought an official play and charge kit for my xbox 360 from Game for £3.99 Preowned.
Comes with a 1 year warranty.
In store only, I believe, as not showing on their web site.
If not on display in store, just ask one of the shop assistants as the guy in Lincoln had to go out back to find one for me.
I know that this may get slated due to "second hand battery life" but for nearly a quarter of the cost of a new one, with a 1 year warranty too, I thought it was worth the risk!!