Lemon

Hi guys, I am new to this forum as am recently an S-Type owner. It is a 2002 model 4 litre V8. I was driving down the M6 motorway when I heard a noise under the car and then saw a piece of plastic in my rearview mirror. When I stopped at the next services the large black cover at the front underside of the car had broken partly away from its mounts and was scraping along the floor. I am not sure what caused this but anyhow, I need to get it fixed, although it doean't look important to the running of the car at all. I am wondering if anyone can direct me to where to find the relevant part? Also I am thinking to change it myself to save money if possible and wondered if anyone had experience of this? Thanks a lot. And good to be here on the forums :)