Seems to be the best price for a XBox Wireless controller. Available online and instore but the delivery is £6 online which is a shame (thanks DHaynes!)
I went in to the Cheltenham branch to pick up a wireless headset, and all the wireless controllers were £19.99. Apparently they're clearance nationwide in preparation for the new model with the improved D-pad. Genuine Microsoft product, with Duracell batteries.
However, the price is now the same for the NEW model! (thanks st0nec0ld!)