Rear panelling out in the boot to gain access to the lights unit. Take the rear lights out, then the 2 bolts holding the top of the rear panel out. Remove the rear number plate and remove the two screws behind there.
Under each wheel arch remove the rear plastic cover panel 2 bolts holding each side. Also you’ll find a fastener up inside each wing holding bumper to wing. If you have mudflaps fitted remove those first. One last thing, there’s a plastic stud in the rear light openings each side, prise these out.
Now you can commence to remove the bumper, the joint between rear wing and bumper is held by clips, gently pull that part of the bumper away, it should come away ok without the need for new clips.
Just to add, have a piece of carpet or cloth handy to place down on the floor to protect the paintwork when you lower the bumper. Another pair of hands is better but not necessary.
Oh and like myself, if you had rear parking sensors these need disconnecting too.
Sounds complicated, not really its pretty straightforward. You can fully see what needs attention under there re rust removal or if it’s just an inspection.