What you have happen is typical of prepayment billing at Powergen. There can be a lot of false credit/debit balances showing on statements because of the manner in which the accounts are billed. Generally speaking a prepay cutomer should be able to go by what is showing on their meter, including any debt set.
Unfortunately on the odd occasion when the meters are installed the correct tariff isn't set to the meter and you end up with the experience you had early this year. Tariff codes should be set when the job is booked as this is how the correct tariff is set on the meter, the billing system automatically sets the correct tariff on the account.
In relation to the agreement to pay of the outstanding balance after change of supply, did you receive a letter confirming the agreement? If not then it sounds like the agent who originally agreed the £10 per month payment didn't set this on the system. As to CS saying they have no record of previous conversations that you have had, if anyone has been onto your account it is logged by the system with the users ID number even if they haven't left any notes. It may well be worth getting a S.A.R - (Subject Access Request) under the Data Protection Act, although this will cost you £10.00. You could write and complain but in all honesty you will probably get a standard "sorry for the inconvenience caused" letter, unless the S.A.R - (Subject Access Request) uncovers something else
