pre payment wrong tarrif

kvdl

Please could someone advise me. I moved into my current address about a year ago and it had a pre-payment meter. I decoded to leave it in as it has proved helpful in controling how much electricity I use. Ihave however just decided to switch to EDF from n-power. The switch-over went smoothly or so I thought until this morning I recieved a final demand bill from n-power for an outstanding balance of £50. When I called them they told me it was a mix up on their end and that my meter had been set to the incorrect tarrif. The whole idea of a pre-payment meter is to avoid this very thing from happening. What do I do now am I supposed to just accept their mistake and pay the money.