Expensive service

I took my Renault Scenic to Kia Wonderwaters for a service. My service plan has just expired. All of a sudden, I'm informed the cambelt needs to be changed. When I asked why they are doing it now that I don't have a service plan anymore, they confessed that the change had needed to be made whilst the car was on the service plan. They just shrugged at me and said if I don't change the cambelt something bad might happen to the car. I got the bill: The labour alone wass R4720 and the total of the bill was R10917.52. I want to know why I must pay so much for a mistake that they made. Surely I paid for the service plan as well and that meant I did not get value for my money. My phone calls and emails to Mr. Christo Robers are either deferred or ignored. To add insult to my injury, Renault SA, who reckons the amount I paid was propesterous, only phone me once a week to hear if I sorted my issue with the dealership. So no help from them too!! I am old client of this dealership and am shocked and dismayed with the treatment I'm getting.