found the car like this when I came to it yesterday

I haven't reversed into anything, and there is no surrounding damage to suggest anything big hit me.

perhaps someone poked it in, perhaps it rattled itself loose.

either way, will it be spendy-spendy to get fixed ? (not sure of my own competence to repair, especially if it means taking the car to bits.

Undo 4 screws (torx) on the underside of the lower bumper trim, bend it back and push sensor back into place.

The sensor is held in a plastic holder with two tabs that keep it in place, these need to be bent outwards to remove the sensor.

The holder itself is glued to the bumper trim plastic.

An easy fix, just have something of suitable size to jam into the gap between the plastic bumper trim and bodywork, that way you'll give yourself room to get an arm in without getting grazed to bits.