Bidvest McCarthy VW Durban - pathetic service

I have a full maintenance plan for my vehicle. My vehicle has been jerking and going into limp mode and when i took it to VW Mccarthy they refused to repair it. They said the problem is with the spark plugs. They stated that spark plugs have been an issue recently due to the fuel that is being used. Apparently the fuel that we use is imported and that causes issues with the spark plugs and they refuse to change the spark plugs. No where in my maintenance contract did it state that there is a specific brand of fuel to use or that the spark plugs will not be covered in a maintenance contract. They went on to say that all the major brands are having a similar issues with regards to the fuel