Unpaid refund from Kelston Motor Group

I bought a vehicle from Kelston Motor Group in October 2017. When I made the decision to buy said vehicle, I paid a R15 000 deposit within 15 minutes, via EFT. Once the vehicle was delivered to me, with an Invoice stating the Balance, I paid by EFT within 10 minutes - in fact the Agent said the email confirming the payment was on his computer before he got back to his office. Two days later, someone from the Grahamstown, EC branch called to say I'd overpaid since the invoice hadn't reflected my deposit. I was at fault for being careless about checking the invoiced amount. I gave him my banking details to do the refund and have heard nothing more from them for the last 6 weeks. I've sent them photos of my bank details, and still they never followed up with me to say they weren't satisfied with the documents. There have now been TWO month-ends (I was told they'd refund me at month-end) and I'm APPALLED at the bad service. I paid them IMMEDIATELY, but I've had no joy from a large company in paying me back the money they owe me. I resent having to do so much work, make so many phonecalls, send them proof after proof of my banking details with no results, and I believe that they have ********* accounting practices. In the meantime, I have bills to pay and I'm out of pocket. What should I do?