Just perusing Ebuyer and came across this gem. Reviews seem decent, worth a punt at £5.99.
Showing as Free Delivery for me.
Info From Ebuyer:
We've all been there (or at least some of us have). On top of a mountain, deep in the woods or by a spectacular waterfall. We want to take a selfie or ring our friends to boast about our achievements, only to find our phone is dead. Crisis! What do we do? We need to capture the moment.
Relax. It's not a problem, at least it isn't if you have one of these handy Xenta solar chargers strapped to your backpack. Just plug your phone, or other compatible device, into the USB port attached to the back of the panel and use the sun to recharge your battery. Simples.
Using the Backpack solar panel
It's pretty easy. Set the panel up to catch some rays when you stop on your hike, hang it on a tent pole or attach it to your backpack. Once the solar panel captures the sun it will turn the rays into energy that you can harness by plugging your device into the panels USB port (cable not included). Under ideal conditions the panel can provide around one Amp of power.
At 19.6 x 29.3cm the panel is slightly smaller than an A4 sheet of paper, very light and easy to carry around. There is a hanging loop for added convenience.
The Xenta solar panel is a 5 watt mono crystalline panel with a USB output port producing 5V/1A.
Mesh cover with zip to rear
Secure straps to hold phone
Works with any USB enabled device