Hi Red Army,
Regrettably there is no way you can leave a supplier without paying the final bill, as of the end of the day clearly the consumption has been used, its just your supplier hasnt kept a stringent check on your usage/monthly payment.
However, if you have a regular monthly payment scheme,by DD, then Powergen are really not supposed to block your transfer to a new company. Energywatch asked all suppliers to stop this practise a few years ago because it was unfair to tie you in like that when you might be financially better off changing due to better rates being available and unlike if you were purposely not agreeing to pay anything, you have a DD in place.
As for your credit rating I know most suppliers dont really do a check, unless you have Prepayment meters, and ask for them to be removed with credit meters being installed, they ask for a security deposit, which isnt applicable in your case.
As for keeping stringent checks yourself I would go with an online account with your new supplier, I myself am with ScottishPower and there online account is excellent, you can produce your statement 24/7, entering your readings online, and you see your usage there and then, you can even do it on a month by month basis if you like, with a monthly dd still being paid for a set amount. Clearly there are other suppliers around with Online accounts, but as I havent been with them I cant really comment, well except for BG - keep well clear!
Once your application has been made, your new supplier will ask you for your readings a few weeks after, once your at this stage, Powergen have definately let you transfer. Your final bill will be based upon the readings. Once you get your final account, clearly most companies ask for payment straight away. However, I would offer to clear the balance over an agreed payment schedule, as it sounds like they messed up aswell not notifying you of the need to increase your DD, if they refuse, then suggest you mention going to Energywatch or speaking to a supervisor.
Hope this helps, and I would definately try a switch, especially online, and see what happens...