Ok, revised and more complete notes as I forgot about some screws you need to remove. Sorry.
I would recommend buying a set of trim removal tools (like
these) but they aren't necessary.
1) Remove the floor of the boot, the spare wheel and the tools.
2) Remove the 2 bolts towards the rear of the car behind the spare wheel that are holding the piece of black plastic trim on the lip of the boot. Then pull the trim firmly upwards. The trim should release. Check that all of the clips are still in the trim, otherwise use a broad flat bladed screwdriver to prise them out. When refitting this trim ensure the seal is properly positioned.
3) Remove the carpet from the back of the rear seat by unscrewing the flat black plastic trim screws.
4) If you have the chrome boot hooks on the floor of the boot remove these otherwise remove the two push button clips on the floor of the boot where the trim comes down and curves round.
5) Now, in the boot, at the top of the side bits of carpet trim (underneth where the rear wings curve round to the rear deck) there should be another plastic clip you can remove. EDIT: And there is also a metal hook through a slot in the trim.
6) Now you should be able to firmly pull the trim towards the centre line of the car. The hidden clips are all along the edge of the boot opening. This bit is tricky and you may well end up with the metal clips stuck in the car and no longer on the trim, use a wide flat blade screwdriver to pry them out. You may also break a couple of tabs off the trim (I did) but you could always glue them back on. Note that the left hand side (as you look in to the boot) is a pain because of the 'well'. I couldn't remove the well from the trim whilst it was in the vehicle so the whole lot required some 'waggling' and pulling. Once out the 'well' can be removed which makes refitting much easier. Also if you have the TV system fitted there is an additional mounting bracket which will need to be removed. I don't have this so can't comment.
7) Slacken rear wheel nuts.
8) Jack car up and remove wheels.
9) Remove torx head screws inside wheel arch that hold rear bumper on.
10) Underneath the vehicle, remove the two (?) Torx screws holding the bottom of the bumper to the vehicle.
11) Undo the 4 (?) cap nuts on each side.
12) Place quilt / towel on floor.
13) The bumper needs releasing from the rear wings. Do this by gently but firmly pulling outwards, away from the side of the car. The bumper will unclip.
14) Carefully remove the bumper from the car.
15) You will see that the rear bumper is marked where the factory fit parking sensor holes would be. Use a 2mm bit to drill a pilot hole then go through with the hole saw from the front. Take care. I would invest £20 in a proper hole saw for this job.
Hope this helps. Paint the sensors otherwise they will look crap! I kept popping my sensors in the oven (on very low ~50 degrees) for 10 minutes or so to help speed up the curing time in between coats. This also meant I could fit them in a day. I bought the Cobra Parkmaster sensors. These are really good. The period between the beeps is infinitely variable rather than the cheaper systems that just have four different intervals. eg. instead of Beep........Beep....Beep..Beeeeeeeeeeeeeeeep the Cobra system goes Beep........Beep.......Beep......Beep.....Beep.... Beep...Beep..Beep.Beeeeeeeeeeeeeeep. Does that make sense?! The Cobra system doesn't have a little screen, either, which IMO is a 'Good Thing'.
