They are made of a quite brittle plastic on the 5 door (not sure about the 3 door) and basically are located by self tapers on the underside and at each end into the wheel arch.
Their brittle nature is why there are so few spare ones around!! - Muppets at tyre centres also had the habit of jacking the car up - forgetting the skirt - and then BANG!!!
Yes, people at tyre places have an IQ of about 10!
If you get a block of wood and cut a groove in it, it fits on to the sill and allows you to jack the car up easily.