Engen petrol attendant puts diesel in a petrol engine

I went to put in fuel at the Engen garage in Norwood Grant Avenue one night only to find out next morning when my car would start that the petrol attendant actually put diesel instead of fuel. I then contacted management who sent their mechanic over to drain the diesel out and put in fuel. However, the engine still did not start. It was then discovered that my spark plugs have been damaged by the diesel itself and was promised that these will be replaced. A day after management changed their minds and came with some excuses that the spark plugs were already damaged prior to the diesel incident and hence they cannot be held liable for the damage and that there is no way that diesel could have caused such damage as diesel actually 'cleans the injectors'. They also claimed that my car has not been properly serviced and that I must provide them with proof of when I last serviced my car, I then produced invoices of my previous service to prove to them that my car was serviced less than eight month ago. I drive an Audi A3 1.8t 2011 model and I make sure that my vehicle is fully serviced on an annual basis according to its service intervals. I have never had issues with my car since I've had it as my car is still in a mint ''second hand new'' condition till this diesel incident. This is not the kind of service I would expect from Engen and this has never happened as I always put fuel at the same garage for years because it is situated in the area where I live.