There are six bolts from the bumper into the rear cross member to remove, accessible from inside the boot. You'll have to remove the rubber plugs to get to two of them. After that, remove the boot lining (that's the "trunk" lining if you're a Yank) and you'll see four bolts (two on each side) that secure the bumper by screwing downwards through the metal wing into the top sides of the bumper. Now go underneath the car
, and there are two bolts accessible at the rear of the bumper, accessible from beneath. Now you need to uncrew a pair of 10mm plastic nuts under the wheel arch lining/mudflap and two which are behine the rear wheels. Fold the wheel arch lining away from the bumper and there's a screw in the very front of the sides of the wing right at the point where it connects to the wheel arch. remove these (one on each side) and the bumper will slide backwards and off the body. Voila. Piece of cake. Remember to disconnect the wiring to the rear sensors and pull the multiplug through out of the hole, carefully removing the grommet.
Re-fitting is a bit trickier, so use a couple of beer crates or boxes to hold the bumper up below the rear as you line up the tabs on the air exits and slide the bumper back.