Can anyone tell me whats the best/easiest way to get to them, I've got one thats come lose, and droped back into the rear bumper, they seem to be all working.

 

I had to replace one off my reversing sensors and found that I couldn't get my hand behind the bumper.

I ended up using the above You Tube clip to remove the bumper..

It is abit of effort but was the only way to do it.

Remember to put something on the floor to put your bumper onto as you don't want to scratch it.

My passenger side second one in dropped inside again. Unscrewed the black trim torx x5 and put my hand up without removing it. I used the Superb boot rechargeable torch as it’s magnetic to attach to the bumper bar. Gave me all the light I needed. Now the pain was getting it to go flush in the hole. Took about 5 goes and lots of twisting and force. It went in and a few more twists it’s flush again. 

I did use ramps to get the car up at the back. And I also have a factory tow bar hole so maybe why I had an advantage on my hatch. I finished it off with some hot melt glue to try and prevent it falling off again. 

Drive the car rear wheel(s) on a brick for extra height.

 

Undo the relevant screws from the bumper underside.

 

Lever the plastic open and wedge with something e.g. block of wood.

 

Reach up and in.

Thanks everyone thats really helpful.

What actually holds them in place ?

If I recall it's the wiring plug that's a finger nipper, it should be easy enough to simultaneously pull from inside and push externally on the sensor to pop it fully out of its clip type holder.

 

Thankfully it's been a while since I replaced a few of mine, let your fingertips feel the design, maybe even shove your phone in and take video to help assess things.