Overcharge with no customer service

Hired a car from Alamo at their OR TAMBO branch. Booked on line as is my usual process. On collection the assistant wanted to charge a higher amount no explanation given. When I showed him a copy of the reservation he acknowledged the reservation and took a copy of the booking and put that amount through on my card with the deposit and all appeared in order. When my deposit was returned the full card transaction was returned and a higher rental processed. Called to query, and was told the desk I got thorough to was Woodford not Alamo although they part of the same company. Sent emails no response. Called the same number again listed as Alamo on the web and was told Woodford cant help, to log a complain on Alamo site - it does not work. 4 weeks down the line still no resolution and no refund of the over charge