Very unhappy

We submitted a claim because the bathroom floor under the sink against the outside wall has settled down about 15mm. There is movement between the tiled wall framing of the build in bath and the bathroom floor tiles. Auto & General sent out a builder who advised that a engineer should come out. The engineer said that the sudden settlement could only be caused by a water leak that washes away the subfloor and he advised that a plumber should come out. The water leak could only be from mains water piping or from the waste water pipes from shower. When plumbers came out they could not detect a leak and our claim was subsequently denied. They now claim that the damages occurred over an extended period and is not new. We took the subfloor up ourselves and found that it was the waste water pipe from the shower that was washing away the subfloor. I just need to add that we bought the house only a year back and started seeing the settlement in the floor in August 2020.