Reapoff on Service

On the 03/10/2017, I went to Fedautor- Ford Kokstad for my Car major service. When they were taking out the spark plugs they broke one of them inside the engine and they told me that I will have to pay the damages of fixing that problem of the broken spark plug. The damage was estimated to around R15000- R16000. My question to them was, why do I have to be the one to pay that even though I gave them my car in a running order and sent it for service not for a broken spark plug. The response was if this is my car I have to pay the damages and costs which is a reap off. I left the car with them as I could not afford to get angry as I was so close.