Start here - and follow it up with a call to your old energy company to claim your money back. Those nasty rapacious energy companies have been hanging onto your money and not repaying it - claim it back!
Claim your credit back http://www.myenergycredit.com/
My Energy Credit helps people who've moved or switched claim money back that may have been left behind with their old supplier. No matter how long has passed, legitimate claims will always be refunded. The average credit balance is around £50.
Choose your energy company from the list below to find out more about the claims process. Suppliers keep records of past customers and will be able to find the relevant information. You may need to contact more than one company to find out who your previous supplier was if you don't have the details to hand.