Refund of deposit

I paid a deposit of about 200 GBP in the Kingston Upon Thames branch when I hired a car from Europcar on 11 Sept 2017 and returned it on 26 Sept. Since then I've been trying to contact Europcar to expedite the refund. It should be a 100% refund as there was no reason whatsoever that they should have deducted anything. Phone calls and emails to date have proved to be totally ineffective.