In case anyone is interested, I thought I'd let you know that Maplin stores are currently selling Camelion Nano (the size of an ipod nano) power packs reduced to £5 from £20. It's 5V 750mAh with usb connector (I've charged mine nicely from a large freeloader 5V solar panel).
I tried it with an arduino nano and rtc circuit, and it works really well, although I haven't timed how long it'll last for.
Thought I'd mention it as they're ideal for lots of little projects, maybe even a mini robot.
Best wishes,
Andrew.