We were assured we would only be billed what we used each month but they are a bunch of rip-off merchants. Bills started at £36 a month and went up to £186 a month. We even got a bill for £186 when the house was empty for two months. Phoned them up and they said they would send proof that we had used that much energy that month and they just resent all the previous bills. I paid what I thought was a very reasonable amount each month and left. Been chasing me for 6 months. They got my mobile and (inaccurate) forwarding address from my landlord, probably in exchange for not cutting off the supply.