I found Atlantic very poor. I was also nearly £200 in credit with them, but eventually after 4 phone calls to customer services I got it refunded - although you should not have to ask for your own money back.
I would call Atlantic again, have your account numbers at hand, and your final readings which you must have supplied to powergen. Take down the name of the person you are speaking to, give them your account numbers, final readings and date of transfer, request that they issue you with your final bills and refund any monies due. Ask them how long this will take.
Alternatively, I would put all the above details in a letter and send it to Atlantic by recorded delivery - this should do the trick.