Contaminated petrol in my bike

I filled my bike with petrol and broke down 5 min later, called the mechanic and he came and fetched me and the bike, in his findings he found that diesel was put into my bike, I went to shell honeydew with the petrol and the findings by the mechanic, I have been to the garage 7 times and they refusing to look at my complaint, they now want me to go pay for the petrol to be tested. When on the invoice from the bike mechanic it explains very clearly why the bike broke down, they have not called me back once after promising to get back to me on the issue. 7 times I have been to the garage, your would think that me being the customer who has a grievance would have some sort of feedback on my complaint.