Dekra report inadequate

We bought a car and the test drive was fine... No issues. Dekra report did not indicate leaking radiator. We Purchased the car and 20 minutes later on the way home. The car started overheating. This did not happen on the test drive because the set route is very short. We Buy Cars refuses to do anything about it. What is the point of the Dekra report. There was almost no water left in the radiator. Does no-one check this...But We Buy Cars get away with this because they make you sign a form taking no responsibility for anything. Very disappointing