Avis Car rental Refund of deposit

Insurance Company provided Car Hire through Avis when vehicle was ******. I had to pay immediately R1 450 to enable me to collect car on 13 December 2018. I returned car on 11 January 2019 (without any damages - it was confirmed by Avis staff member receiving vehicle; with full tank of fuel - confirmed by Avis staff member receiving vehicle; no etag costs - this had to be confirmed the next day by Avis staff). I demanded my deposit to be refunded that Monday, but staff member at their desk confirmed to me that they will refund latest 18 January. I phoned the number reflected on the envelope with their documents on 18 January and a promised is made that I will receive the money the same day in my account. No results! I have sine phoned Avis on various occasions and then only to find myself holding on forever. I have also sent mails to their customercare division who responds back with a generic mail that my email will be attended to. It is now 5 February and still they owe my refund. I now demand interest with effect from 13 December as they are sitting on my money which I could have placed in my 30 day call account!!! Please help!!!