I'm with Powergen and they have a different tariff that they don't realy promote - I think you have to ask them for it. Its called Online 10.
Read about it on MoneySavingExpert site and rang up. First of all I asked about freezing my current price, they gave me different options. Never mentioned this other tariff so I asked. They said yes I could go on it and it would be better than price freezing. Will work out at a yearly £250 saving for us.