Shell Garage attendant pours diesel in petrol tank

Hello Peter.. My name is Justice. I am frustrated and upset by the level of service and care shown by William the petrol attendant at Shell Garage in Pretoria North on Rachel de Beer Street. While at the station this past Friday, I had the unpleasant and very costly experience of him pouring diesel in my petrol tank, and subsequently the repair cost of draining the diesel and replacing the petrol filter. I have been trying to get in communication with William the attendant to no success. Please advise as to how I may escalate my issue to the right people who may take it as serious as it has affected me and may happen to someone else.