Anker have sent me a discount code for their black external mini battery rated at 3000mAh.
Anker Astro Mini 3000mAh External Battery with PowerIQ™ Technology - black version available for only £10 at amazon using code.
I have the silver version and it came in really useful whilst I was at Glastonbury when I needed a quick charge and no power sockets available.
Build quality was exceptional and unexpected at the price I paid. It has held up well to being flung about in my backpack over the weekend.