do not take responsibility for defects

I have a Tata superace and Monday morning would not start. Gets taken to Edenvale Tata service station. The prognosis is that the diesel I am using is dirty. Now I fill up every time at Sasol filling station Brits. Tata will not take resposibilty and clean the dirt and I have to pay R5200.00 for the work done..The vehicle has 54 thousand Km and is still under warrantee <br>