Bad service

My car had Engine light on and I booked it to be checked, they told me the spark plugs were damaged because of the quality of petrol I used and I have to pay for it even when am having both warranty and service plan . I asked a simple question did he took the petrol for testing? , No. The very same petrol he claim it's not quality I still have to drive the car with , I also asked if maybe we should report that to Astron Energy , he it's all the petrols. We are in trouble, bad service, a robbing game to customers.