Mercedes Benz poor service

Hellopeter I bought a Mercedes Benz car at Mercedes Benz New Vaal garage last year May so December I had problem with the car it was not performing well I had to call Mercedes guys to come and assist fortunately they sent their nearest branch which was Inyanga motors to come and fetch the car and did come and take. From than I tried so hard to get a courtesy car which they ended up providing. I have been in contact with them trying to figer out how far are they with the car so they were saying it had because their diagnosing match is saying the car is fine there is no problem but they can see themselves that the performance of the car is poor. Than I had to wait until now they sent me an email saying the problem is the engine so how water infiltrates inside the engine that is why the poor of the car is poor and unfortunately they can't help because it's an external impact I have to be responsible for that. I'm finding it very strange to understand how could it be possible that engine failure is defined as external impact and how did they come to conclusion that it's my fault coz I just bought the car last year and it under warranty.