Warranty Expiration Complaint

This is regarding 2011 Chevrolet Cruze. In July 2015 I contacted CMH GM Umhlanga before the 90 000km service. I asked for a quotation from the service department. I was contacted with a ridiculous quote of R 8300. I then asked what my options where to get it done for less. They told me an RMI approved workshop would probably get the service done for a bit cheaper. I asked the person if this would not cause issues with my warranty, and the answer was NO. I went to RMI approved workshop, Forest Drive Auto and they quoted me R5200 and had the service done with them. My warranty only expires in June 2016 which I was happy to hear, but then came the dreaded comments about the warranty being void because I did not have it serviced through GM even though I was advised to do. I am disgusted! I have been having endless problems with the vehicle. Strange thing about the whole RMI approved debate is that I contacted Emond Auto in Durban North to get advice and they advised me to speak to Rex Diff & Gearbox where they send their vehicle to with transmission problems. They are also RMI approved, so please tell me what the difference is exactly. I am demanding my vehicle be fixed under warranty.