Avis withholding deposit.

I have rented a vehicle through my insurance with Avis with Rental Agreement no:E ********** 90. The rental period was from 8November until the 23 November. I have paid a cash deposit when I collected the vehicle. I have waited for more than 14 days before I enquired about the refund of my deposit. Every time I contact them I get transferred and put on hold for long periods of time and their *********** call centre staff never followed the matter up. On the 12 December they informed me the matter was actioned and that the money would reflect in my account the following week. It has been seven days later and when I enquired this morning they informed me that it was only processed on the 20th of December meaning that I would need to wait another 5 to 10 days to receive my money. This is very frustrating and it seems that they make excuses every time not to refund my deposit. I feel like I should not be fighting with them to return my money and wait on a bank to release the funds as I paid a cash deposit. When I requested that the escalate the matter their *********** staff said that they will but never did and they also never followed up with me. This behaviour is absolutely disgusting from Avis. I hope they can refund my money immediately via Special clearance.