The Eco Travel Network has again loaned its mobile charging station to Hay Festival for the next 2 weeks. This will be located on the main site (ask at the entrance) and will provide 2 x 13amp sockets and room for two electric vehicles to park. In case another EV turns up badly needing a charge, it might be polite to leave your mobile number on your vehicle so you can be contacted and able to negotiate charge sharing with other thirsty EV's.
If you are staying locally for the Hay Festival, you might like to hire a Twizy to get to and from the Festival site. Details here.