Rediator leaking 18 months after replacement

Last year January I took my car for a service to Renault Melrose, it was a Friday. The next day after the service the car started overheating, which I found surprising. I took it back to Renault Melrose on Monday to report the overheating. I was informed that the head gasket of the car had blown, that I will have to pay R24 000 fix it. I requested that they borrow me a car because my home is in Centurion and it was late for me to make alternative arrangements. I was told that they don't have any car available to lend me, so I decided to fetch my car and drive home. That Tuesday I took it to CMH Midrand, I was informed that the head gasket had not blown but the radiator and other parts need to replaced and was invoiced R17 000. These were replaced and I paid the R17 000. I sold the car to my cousin two months ago. It started giving him a problem and he took it Renault Melrose and was told that the radiator is leaking and needs to be replaced for R 6 500. I contacted CMH Midrand and was told that the warranty on the replaced radiator was for a year and they will not replace it. That means I have to pay again to replace this radiator,1 year and six months later. This is unacceptable