Refund on a car hire

Very difficult to deal with this car hire company when it comes to refunding your deposit for a car hire. It is almost a month that I returned the car to them. This was a courtesy vehicle from my insurers when my car was out for repairs. I paid a deposit and they went further to deduct money from my bank account without sending an invoice or explaining the charges. You try to call them they don't answer, hiding behind Covid-19 but their operations are open and running, They tell you to send an email and they neither acknowledge or respond to your query.

Surely they are not a car hire company but a bank because they hold on to your money as long as it suits them to make money from the interest over and above their exorbitant hire rates.