Holding deposits hanging for 3 weeks

I rented out a car at Avis east London airport on the 11 Oct 17. To date I have not received my holding deposit. I have called in to ask what happened to the money, the assistant lady mentioned its paid.

My bank has confirmed no payment has been made. Avis is very unrealiable when it comes to customer service. I will never use their business if we always have to fight to get our monies back.