Carltex - Krugersdorp poor petrol with traces of diesel

I have been pouring petrol at your services nation wide and i never experienced any petrol tampering, But to my dismay my car has traces of diesel thrown in it and this information I got it from NTT Volkswagen Krugersdorp Mechanical Manager Micheal. I went back to carltex garage owner at krugersdorp and he was shocked at the sample I gave him from VW. Then he said he will call head office to check what is happening. But in the mean time what do I do because my car is faulty and i use it to get an income. I have to pay R3 387.45 while the so called tests are done. I will stay away from all carltex garages in future.