I've finally got round to doing this job. Firstly, the instructions above are excellent - not just for bumper removal, but for nearly any dismantling of the car. It seems rear bumper removal is a lot less painful than removing the front bumper, but there are still a couple of sticking points. There is a screw and captive nut each side the same as those used at the front. These are mid way down the wheel arch and will most likely be best removed with an angle grinder with the wheels removed for access. There are also a pair of 10mm nuts and bolts just behind the mud flap. Again, best removed with an angle grinder. So before starting, I suggest you source the torx screws and captive nuts (part number: C2C35071) and a pair of 10mm nuts and bolt. Also, it may be worth getting hold of the plastic trim fasteners (part number: JC2S 9354020) as used to hold the bumper to the car at the lower edge of the central rear section of the bumper. The torx screws are no longer available, but I got hold of some stainless steel ones on eBay from Germany.
The first thing to do is to remove the boot floor and side trim pieces and rear trim piece. The floor is removed by two torx screws where the luggage tie down rings are. The side pieces are kept in place by one upwards facing plastic trim 'thing' and the lip of the rubber boot seal. The rear trim piece in kept in place by 2 torx screws, again near the luggage tie downs and also 2 plastic trim fasteners. This will give you access to 4 screws pointing backwards into the bumper at the rear most lip. I didn't have any trouble removing these, but they were showing signs of corrosion, so could possibly prove more troublesome on a different car.
With all the trim removed, locate the wire that goes through a grommet to the bumper's parking sensors. This is to the side of the large rectangular electronics module on the right hand side of the boot (as you look into the boot) and between it and the smaller module, protected by plastic. The wire runs down into the small well below these modules and out through the grommet. Disconnect it at the connector located above the smaller module and leave the slack in the well so that you can pull it through when the bumper is removed.
Next remove the 4 plastic fasteners at the lower edge of the central bumper section. With this done and all the screws removed, the bumper should come off with a tug at each side to remove it from the clips. This is the same action as with the front bumper. Make sure you can support the bumper so that you can remove the grommet and pull through the wire and connector. Job done!
Now for some photos:
Position of the torx screw within the rear wheel arch.
If you prise out the reflectors, then you can see through to the captive nut and can admire the corrosion.
This is where the grommet fits and the wire gets from inside to outside.