Double charging for fuel

Please do not use this car rental company. I filled up the tank at Cape Town airport on returning the vehicle and they later charged me for an extra 10Lts. When I queried their customer service they were not interested. I sent them the card transaction for filling the tank at BP Cape Town airport and they even got the airport wrong. Didn't care and just refused to refund. When I called the branch at CT airport itself, they apologised and promised to refund me the money. A month later and nothing has been done....useless