joannea751,
The first thing to highlight here is that Npower, or any energy provider for that fact, can only "back charge" you for the previous 12-months. If they are trying to charge you for more than this then this is not allowed and you need to speak to energywatch about this. (Tel: 08459 06 07 08)
With regards to the prepayment meter, this is a real hot-topic at the moment as the government and Ofgem (the energy regulator) are being pushed to address the fact that prepayment customers are paying approx. £250 more per year than people on normal credit meters paying by direct debit.
It really doesn't make sense that providers put you on a prepayment meter that is more expensive at the same time as asking you to repay debt.
If you wish to email me your contact details I will ask someone at Npower to call you and look into your case.
Email: scott.byrom AT moneysupermarket.com
Hope this helps.
Regards,
Scott Byrom
Utilities Manager
moneysupermarket.com