Poor product

Filled my tank with Sasol 10 diesel earlier this week. Immediately my car felt sluggish, wouldn't idle and made a loud knocking noise. Today I filled up with Shell 50ppm diesel (supposedly inferior) and my problems disappeared. When confronting the owner Mr. Imran Ali he pleaded innocence and claims he has had no other complaints. My sole deduction is that either he filled his underground tank with dud diesel from somewhere or Sasol gave him a inferior batch. He lost his temper when I intimated the above and I left.