Bingley,

If you have minimal skill with a soldering iron or are open to building a new skill take a look at the MintyBoost from limor fried. It will charge most any smart phone of device that can be charged from the USB port of a computer.

The following is from the device web page. I can not recommend the device enough. There is even a solar version on the site.

Learn to Hack, Hack to Live,
Jim

(Hack as in A clever or elegant technical accomplishment)

http://www.ladyada.net/make/mintyboost/

Minty Boost: Portable USB power

Overview
This project details a small & simple, but very powerful USB charger for your mp3 player, camera, cell phone, and just about any other gadget you can plug into a USB port to charge! (See below for compatibility tests)

The charger circuitry and 2 AA batteries fit into an Altoids gum tin, and will run your iPod for hours! 2.5x more than you'd get from a 9V USB charger! (See Process for math/calculations) You can use rechargable batteries too.

Specifications:

Version 3 now works with iPhones and newer iGadgets!
5V output @ 500mA output
Fits in your pocket
Recharges just about any gadget with a USB cable
Uses any AA batteries!
Upgrade to C or D cells for a mega battery pack

Some numbers...

iPhone (all types): 3/4 full recharge
iPod video (tested, using alkaline batteries): 3hrs more video (1 full recharge)
iPod mini (tested w/rechargables): 26 hours more (1.5 full recharges)
iPod nano: 4 full recharges!
iPod shuffle : 60 hours more , 5 full recharges!

This project is suitable for beginners, some soldering tools are necessary but even if you've never soldered before it should be pretty easy. You can etch a circuitboard and/or breadboard this up, or simply buy a kit from the webshop.