Was intrigued to be told by the water company that they couldn't use an admitted overpayment of hundreds on one account here to wipe out a bill of a about fifty on another account here; both accounts are in the same name and at exactly the same address. That's taking the water.
Now, I can't see why you couldn't sue Southern Electric for the return of payment made under a mistake of fact. It's not much good their saying that their terms of business don't allow repayment or that it's to be construed as advance payment. The short answer to that is "What would happen if I terminated the contract? On what basis would you hang on to monies which were for a service and supply you could not provide?"