bplanky:
Is this a domestic or business supply?
If it is a domestic supply then NPower should observe the provisions of the Code of Practice for Accurate Billing. If the circumstances qualify only energy consumed in the last 12 months can be billed.
http://www.energy-retail.org.uk/customerbilling.html
If you think you qualify raise a formal complaint strictly in accordance with the NPower complaints procedure setting our your grievance. When/if you are not satisfied with the final response or they breach the very tight deadline for their response you are entitled to refer the matter to the Energy Ombudsman.