Hi Mel, if you moved into a property which had a prepayment meter sometimes you end up paying the debt of the previous client. If you have always been responsible for your own prepayment meter then a debt should not have built up, but if you switched to prepayment and had an oustanding debt on your old credit meter then you will still be expected to pay it off. If you switch supplier I doubt any new supplier is going to take over the debt. So you can go back to your old credit account records and see if there was a debt on it, and if there was not you can challenge SP.
Don’t understand how you can build up £300 debt on PAYG. When your credit runs out the power stops. Or did you have a credit meter which you swapped to PAYG? In that case you have to pay off your debt slowly through PAYG.