Still waiting for my refund from last year

I rented a vehicle on the 24th - 29th December 2014 and I'm still waiting for the refund. They claim that I used the car past it's service date and, it's even stated on their checklist that it was due for service when I took it. I made them aware before I took the car and they acknowledge that I can still drive it for 1500km and I only drove it for ****m. Every time when I try to talk to them they say they'll come back to me in an hour but, no one is contacting me. This is not happening for the first time because I had to go to their head office to get my refund the last time. I was assured that it won't happen again because it was a mistake on their billing side but it is happening again. This is really bad service from the company that is starting to operate. Will never ever use their service again and will make sure that everyone knows about how they treat their customers.