Overbilled and no refund despite calls/mails

I rented a car from Woodford for 3 days, had a daily limit of 100km's per day and used far less then the 300 allowed. They overbilled my credit card by about R600 with no invoice being supp****, ignored a followup message sent via their website and when called agreed it was a mistake, promised to do refund and give me status. Its now more than a week later, they still have my money and no feedback. The service really sucks!