I know this wind-up phone charger's old. Hell, you know any gadget's old when it's photographed next to the Nokia 8210, but the festival season kicks off in 2 days with All Tomorrow's Parties and if you're quick you'll be able to get one of these delivered by Friday. For a tenner, it'll give you about a minute of talktime for every minute of cranking - perfect for emergency 'where are you?' calls; not perfect for topping up the battery to max. You can buy it with an adapter that'll work with most phones from here.
Don't you get to stay in proper rooms at All Tomorows Parties? Surly you could bring your charger along.