Please return my money

In August 2014 I transferred N$50000 to my Diners Card account to ensure funds available during my holiday September/October. On my return from holiday there was a remaining credit balance of N$ 33300. At first I was not too concerned until I discovered 6 months later that Diners have not paid a cent in interest. NOW THE FUN STARTED! A month ago I requested by telephone that they return the credit and cancel my membership [I have been a member for 30 years]. The telephone conversation security verified my identity and my account has always been settled by means of a direct debit - so my banking details are known. Diners insisted that I had to send them a copy of my bank statement indicating the original transfer. I mailed them a copy of my bankers original transfer advice. A month later and in spite of a number of emails, I have still not had my funds returned. Diners have sat on my money for seven months, not paid a cent in interest and are now giving me the run around about returning MY MONEY!! They have security verified me , for thirty years they have had my bank details, I have sent them proof of the transfer, it is my opinion they are unreasonably delaying returning my funds