Anker PowerCore 10400 Portable Charger with PowerIQ is £14.99 at Amazon.
* Two USB ports
* Can charge at 2.4 amps per port or 3 amps overall
* Input: 5V / 2A
* Anker 18 month warranty
* Comes with Micro USB cable and travel pouch
Seems to be the cheapest its been according to camelcamelcamel, great price for a decent sized power bank from a reputable brand.