As an update I have checked the letter for wording and it states, "if we haven't heard from you by the time your first bill is due, we will have to change your account back to our Standard Plus tariff, which is more expensive." To me the wording of this specifies that they will change it back upon the due date of the first bill, but in no way specifies that they will backdate the previous energy used onto the more expensive tariff.

Hey all, This is my first post although I have used the site before (lurking in the shadows) for advice on a lot of different things and now I have a query that I can't find an answer to without asking it myself! I moved into a property back in the end of September 2012 & unfortunately I've been having a lot of trouble with Spark energy. I have contacted them on many occasions as I was trying to receive a final bill in order so that I could switch to another supplier. I've now finally managed to receive a bill from them, however the problem I have is the fact it seems they have changed my tariff retrospectively. I was in receipt of a letter dated at the end of March that stated I was currently on the best tariff, however to retain this tariff & complete account setup I would have to provide either a security deposit of £150 within 7 days or provide my DD details or I would be switched onto a worse tariff (the standard plus). I didn't do this as I have been trying to leave them (I was wary in case it meant me having to chase them for money after I had left). However I have now received a bill which states that for my usage between September up until today, my account was on the worse tariff which makes a difference to the bill of about £800 (which incidentally is around half the bill's cost). I've no problem paying for what I have used, however I was just wondering if anyone could advise as to the situation and the legality of the tariff being retrospectively changed to the worse one? Any help would be greatly appreciated.