Bad serviced cars

I purchased a car at we buy cars march 2024 according to the dekra report it has no faults. Our first drive with the car everything seemed OK but when I got home I noticed a leak did not pay that much attention to the issue. Took the car for service within a week I was then told the gear box is a problem and it needs to be sorted out. I called we buy cars as the car did not engage in reverse sent a quotation because I had to buy a new gear box to fit the car paid the workshop. I sent them a quotation they declined my application for their warranty because I did not even drive the car for 6 months it had an issue. Don't buy cars at we buy cars. They are not properly checked