Mercedes Benz refusing warranty liability

We have been having issues with a Mercedes Benz C200 bought in 2015. The car is still under warranty. Last year my wife had an issue driving to work were the car lost power and started vibrating. She managed to make to work, driving at a snail pace.

A call was logged and Mercedes send a callout technician to collect the car. The car was returned to my wife end of same day because "there was nothing wrong..."

Early November 2018 the car started showing same issue. I took it to Mercedes Benz Menlyn. A day later they called to say I should collect the car because there was nothing wrong with it. "Fortunately" as I was driving out of Mercedes Benz Menlyn the issue started again (weird noise and vibration) . I called the Mercedes Benz technician to the car. He then agreed that indeed there was something wrong. He connected their diagnostic unit and said it was injector 1 & 4. The car stayed at Mercedes Benz Menlyn a few days and they called that we must collect it because it was now fixed.

A few days later issue started again! Took the car back to Mercedes Benz Menlyn, they kept it for a week and test drove it numerous times. The car was returned because it was now fixed.

On Wednesday 05 December I returned the car back to Mercedes Benz Menlyn because it was exhibiting the same issue. A day later I got a call from Shaun to say the issue was due to "contaminated" fuel the fuel pump needed replacing but Mercedes Benz will not cover "as they did not put contaminate fuel" into the car. What a stupid statemented . Did I put the contaminated fuel into the car? Do I manufacture fuel?

1. What tests did Mercedes conduct, to conclude that the fuel is contaminated. Please share the report of this test 2. How come the “faulty” fuel pump is not picked up the car`s own warning system? 3. Since this is the 4th time the car is back @ Mercedes Menlyn, what assurance do we have that the problem has been diagnosed correctly this time?

Mercedes Benz is simply finding an excuse to avoid liability. They know very well that no consumer can first "test" the quality of fuel and a garage before refueling.