Credit due to me - still not paid in over 6 weeks

On 17 March 2015 I took my car in for service at this branch. Besides the service being terrible (they did not complete everything in one day, so I was asked to return my car the following day, after I collected it, there was still an issue with the electrics), the amount I was asked to pay for the service was R283.76 too much. They said they would credit me. To date I have not received the credit despite giving them my banking details. After calling them a few times to follow this up as well, no-one has bothered to call me back