When I got the car one of the sensors (2nd from left) was partly pushed in. Worked fine. No it's gone - assume it's dropped inside as constantly firing on reverse. Had a quick look under bumper - Is there a way to get access to this?

 

Bumper off. Not too difficult just quite a few torx head screws. Reasonable guide here: http://www.briskoda.net/forums/topic/223019-removing-rear-weights-mk2-fabia-vrs-tsi-hatch-guide/

As Phil has said bumper off job, not that hard to do really. As your pulling the bumper off be careful of the wires, they are only shortly so will probably need a second hand whilst you fiddle about putting it back in/ taking it off.