The windscreen is suppose to stay freely on the bodywork of the car so that any stone would bounce off, unless it is a big hit and the shock wave would actually cause immediate cracking.
I didn't even know when it hit the car, and most of all, the cracking started progressively, with about 1 cm per hour until it finally went all the way to where it is now. If it was properly installed it would have just stayed where it was. My other Stilo received a stone so huge in the left down corner of the screen I actually saw it coming, 3 cm to the corner of the windshield. It chipped it, but only at the surface, and it stayed there, didn't move an inch. The other windshield on my Stilo received an actual bolder from somebody who purposely hit the car. Again, it cracked the surface but it stayed there.
If they don't want to admit it is a fault in either the windshield or in their mounting, I will get a professional written opinion and sue them. We will see how they react to that. Even for 120 pounds.