Europcar don't return deposits

I was recently provided with a courtesy rental car by my insurance whilst repairs to my vehicle were being undertaken, the rental car was provided from Europcar. At the time of receiving the car I was asked to provide a R1600 deposit on my credit card, which I was informed would be returned 24-48 hours after return of the vehicle. At this time I was asked to please fill the vehicle with fuel prior to returning it to avoid a “refuelling charge” Once the repair was complete I contacted Europcar to return the vehicle and filled it up with fuel immediately prior to returning it. 36 hours pass and no refund from Europcar, so I call them and was informed that it would take 5 days to process the refund, and that I would be charged over R220 for refilling the car!? The vehicle had a quarter tank of fuel (+/-12L) prior to my refilling it with an additional 37L Europcar is now claiming that they managed to fit an additional 13L into a car with a 45L tank (VW Polo) , making the total capacity of the car to over 62L according to Europcar. Needless to say VW laughed at this suggestion, I was told the filler pipe “might account for 3L”, but certainly not 17L that Europcar seems to be claiming. On querying this further with Europcar I was told they need another 7 days to “conduct an investigation” and then a further 3 days to process the refund of my money which they still have all of. This is entirely unacceptable, Europcar kindly return my money immediately or pay me interest on it at the rate of my choosing.