KIA service centre PINETOWN

We have a Kia Rio which is on a service/maintenance plan and warranty. Vehicle just on 55000km. We had a problem with starting - taken to service centre on FRIDAY and was fetched same day as we were told car is ready. However we had same problem over the weekend and had taken car back. They now are charging a fee of R816 for cleaning a spark plug. I find this unacceptable as the problem was not sorted out the first time around. Secondly I was not made aware of the charge before any work was done. Pathetic Manage and service consultants.