Their diesel contaminated with water

I most of the time refill my car with diesel at Spargs Kei Cash Superspar in Mthatha. On the 24 October 2017 between 10 & 11 in the morning I went to fill up my car and when I arrived there was a fuel tanker filling the garage’s tanks. The petrol attendant called me to say I can come to fill up as the fuel tanker is about to finish. The following day when I was starting my car in the morning to go to work, the car could not start, tried several times but it couldn’t. It was towed to NTT in East London where it was discovered that the diesel is contaminated with water (sample available). I went to Spargs Kei Cash Superspar in Mthatha to lay a complaint but the manager, Michael disputed my claim saying that they test their tanks everyday so their diesel doesn’t have water in it and he said I must go to the manufacturer of my car because my car could have a leak. I explained to him that my car has a good service history and was from service on the 19 October 2017 and I have been driving my car all along without a problem until I went to fill it up again. My questions are where was the leak all along? He says they test their diesel but I am pretty sure that particular time the diesel was not tested when I was there to fill up because the fuel tanker was still filling their tankers.