Terrible assessing

Sectri Auto body was given the job to install a sump guard / splash tray for me due to a stone hitting and busting my oil sump. It was installed and I took a drive to Durban just over a month back. Upon returning to Middelburg on the N3 Pietermaritzburg highway the splash tray that they installed had flown off. I took it to them a week after and they then claimed I hit something, which wasn't the case. They then claimed that because there was a scratch on my front bumper that that was the cause. I explained to them that those were there even before the incident. And I have photographic proof. Also when the assessor David Wepener asked me if i want the bumper resprayed I said no as there was just stone chips and the marks underneath that you cannot see easily unless you look under. They went on to speak to their Assessor buddy David Wepener from Kingprice and he agreed with them. I showed him the photos as well and he gave the same ******* as they did. They assume on the basis of a scratch. They cannot pass the buck here and on a basis of assumption not replace my sump guard. My underneath and oil sump is now exposed for the last few weeks and I am driving with risk of damage. I want my splash tray to be replaced due to terrible workmanship and I am not going to bear the costs. How can it just fly off like that???