*** UPDATE - OK, so Zavvi have teh official ones cheaper - http://www.zavvi.co.uk/zavvi/9995971.product
(which is already posted) - my bad. ***
Just pick one of these up from the Colwick, Nottingham 'Tesco Home' store today.
Marked on the shelf as £12.97 - took it to the checkout and was charged £8.97!
I can't find it on their website and have only found it online on Play.com (http://www.play.com/Games/Xbox360/4-/10806016/Exspect-Play-Charge-Kit-Black/Product.html) for a very reasonable £7.99 delivered, but they are currently out of stock - so this could make a good alternative.
(Apologies for the lack of images or direct link - but they simply aren't there!)
If it helps then it's itemised on the receipt as 'XBOX360 23568',
barcode number '5060088523568'
(Also apologies if this is already here - I did search and could not find)