Poor workmanship on offer and ****** at work

I took my car to cartime secunda for a major service ( biggest mistake of my life). On my way home after collecting the car, oil pressure sensor warning light came on. I notified them the about and I took the car back to them the next day, they diagnosed the car and told that nothing is wrong with car and gave back the car. Upon doing my own diagnostic I found that the oil used was dirty and the oil filter used on my car was not the right one for my car. I brought new oil with permission from "Solomon" but now they refuse to refund me. I did contact their head office but they are unequivocally useles as the secunda branch. This service centre is a **** and I wonder how are their RMI approved. I even doubt that the other parts were even change.