Disappointing service

Very disappointed😡 My vehicle was jerking when going uphill. Upon inspection I found out that my spark plugs were completely black and burnt. When I reviewed my service schedule it was ticked that spark plugs were changed at the 105 000kms service. I am currently doing 119 000 so I found it very hard to understand that the spark plugs were in that condition. I had to buy new plugs with my own money at Goldwagen. When I went back to the dealer, my service advisor Andrew insisted that plugs were changed which I find it hard to believe, how would he know? Is he a technician? did he witness the service being done?. What he told me was that the cause of spark plugs being like that would be would be that I put contaminated fuel. Which brings the question of how does he know? Is he a technician? What proof does he have?. It's very sad because when I chose an extended service plan for my vehicle I was hoping to get it in excellent condition through a reputable establishment such as this one, only to be **** to and not doing the correct procedures to rectify the problem. I work at this industry and I know that this is no how you handle a situation like this, it's disgusting that Andrew said he has 16 years in this industry but fails dismally to show empathy and remorse and do the proper investigation or even try to resolve issue. Theres one I know is that Alpine Volkswagen Pinetown has lost my respect and my trust in them.