Ian123

All now fixed. The sensors can be replaced without taking off the bumper. In my case the actual faulty one was on the corner, just take the 6 visible torx T25 screws out of the lower side and corner of the wheel arch liner and put a hand in the gap; unclip the sensor (took a bit of effort as the clips are strong and difficult to get purchase, see note below). Then withdraw and unplug. Plug in the new one and refit. A bit of silicon grease helped the new one in as it was a tight fit.
The inner front one was damaged but still worked; but replaced just in case. Can be accessed thru the grill with small fingers.
For both a bit of strong nylon string looped around the clip and pulled from below gently allowed the sensor to be pushed out, on the one in grille it was simple but for the one on the corner fed it up thru a handy slot in the plastic.
Overall time to change about 10mins each one.