Cant get my refund....

I had cancelled my travel booking on the 7th September 2017 and have been told that my refund will only be processed by the 3rd October 2017 after being told by the travel desk that my refund will be paid no later than 21 September 2017. Now I have to pay the large interest charge that the bank will incur on my credit card at the end of September and Fly mango are probably cashing in on the interest that they have received from my funds for weeks. This is totally unacceptable....