Unfairly treated

Im so upset and disappointed with this BP garage Goodwood on the 16th around 10am I went there for fuel and I asked the attendant to check water as well and he did little did I know that the guy did not close the radiator and while I was driving I noticed the smoke and stopped immediately and we checked only to find out the lid is open and the water is all over I went to the garage to report they checked the receipt and footage and confirmed that they can see the attendant opening but not sure if he closed they asked me to leave the car with them and I did now they are telling me to pay for parts that the mechanic will need them they will cover the labour… I have lost money already by towing the car, my business is on stand by I paid for someone to transport my child to school as well…. Why should I pay the part while its not my fault? Can someone please assist me on this case.