Builders warehouse Newcastle

Builders warehouse Newcastle I purchased Trim Tech edge trimmer which burnt out the motor after just 1 use. Took back the trimmer to the store where it was booked in for repairs(a month after purchase) When booking in the trimmer I asked the assistant helping if it's OK for me to remove the accessories ie handle and cover, which she agreed was fine. Upon inspection by the repair center I was told that I would have to pay for the repair as it was negligence that caused the motor to burn. The negligence being I did not have the cover on. The same cover which I remove in store took back home with when booking in the trimmer. Store manager Andrew was of no help, he is rude and had no customer care responsibility towards me. Instead he accused me of being negligent and raised his voice at me when he didn't agree with what I was explaining to him. I'm no out of pocket on the trimmer that I have used once since builders warehouse and their manager Andrew refuse to listen to my side of the story. Andrew insists I was negligent and he cannot do anything about.