Chez2681,
The issue here is not whether you should go on a prepay meter or not. The issue is that they wrongly billed you for some time.
Although it's your responsibility to ensure that your reads are correct on bills, it's also NPowers responsibility to ensure that the collected, or at least attempted to collect accurate reads over time. Did they do this?
Aside from this, they cannot force you to go onto a prepayment meter unless you are completely unwilling to co-operate.
If i were you, i would write a complaint letter (recorded delivery) to the customer services director (you'll get his/her name from the npower website) explaining the situation. If you explain that you are happy to pay what you owe, but that £389 is a ridiculous amount to pay, and would like to work out an easier payment plan, they're more than likely to help you.
Explain that although you should have provided meter reads, Npower are also at fault for having not collected accurate reads in some time.
Also, explain you do not want to go on a prepayment meter. These meters are expensive to fit, and expensive to maintain, as you get charged higher standing charges.
Lastly, hint to them that if they do not provide a satisfactory compromise within their complaints guideline time limit then you will be contacting Consumer Focus.