I recently replaced the rear bumper on my V40 - it's easier than you might think and perfectly possible to do on your own. I presume the procedure is very similar in an S40. You'll need:
8mm spanner
10mm spanner
14mm spanner
Crosshead screwdriver
About 30mins to remove it, a bit less to refit
Access is obviously easier if you can remove the boot carpet but you don't need to. Start by removing the black plastic trim panel - there are 4 clips along the top that will need just need to be lifted straight up:
Yellow circles show where the clips went into the car.
Then remove the single (plastic) screw on the offside, which holds the carpet to the rear of the car:
You'll need to remove the sub/amp from the offside too - having pulled back the carpet on this side (don't bother trying to get the speaker grill off) just undo the 4 screws, pull it out from the top then lift the whole unit out. You can unplug it if you like.
Next job is to undo the 8mm bolts (green circles) holding the wheel arch trim to the bumper and the crosshead screw (yellow circle) at the top :
You can now unclip the side lights - prize them out of the bumper, then unclip the wiring so it's free from the bumper.
Back inside the car, you should now be able to pull back the carpet enough to see all 6 bolts holding the bumper on - 2 on the side (yellow circles, one immediately behind the speaker that's just come out), 2 facing down in the floor of the car (green circles) and 2 more pointing to the back of the car (yellow circle):
I undid the 4 smaller 10mm bolts first, then the 2 14mm bolts - having done this the whole bumper will slide straight off: it won't fall off, but doesn't need much persuasion.
The bolt circled is one of the side-facing ones, just be careful not to scratch the paint as you pull the bumper away.
Refit in the reverse order. I used ACF50 on much of the metal work to help stop any possible future corrosion (as you can see, there's none so far - not bad for a 15 year old car) and some contact cleaner on the side light connections.