Engen not taking ownership of attendant negligence

Sunday I went to refuel and had my water and oil checked, the attendant didn't close the lid for the radiator/water tank which was discovered after my engine started overheating and steam came out from the bonnet, I contacted the customer care who passed it to the relevant parties, after one day I am told that they don't accept any liability, because of a video footage showing the attendant closing the lid, I disputed that since how does a video show if the cap was closed properly or not and my car just came back from service and no fault were identified, they didn't even bother asking the Engen garage that I went to after my car over heated, I sense that my complaint is not being taken seriously and they do not want to pay for the diagnostic test required by my insurance. My emails enquiring about the outcome go unanswered.