Well, that was a failure....
Sensor is painted, but I couldn't get the bumper off.

The four nuts are very awkward to get to, but something else is holding it on. And I wasn't helped bay one of the four nuts was 15mm and the other three were 17mm. Wasted a lot of time trying to sort that as it was all done by feel.
There's a pair of 16mm bolts that look as if they go in the right direction, but they are heavily loctited.
The WIS says two nuts hold the bumper on, with two more added when the towbar is fitted. No mention of the 16mm bolts.
It's all back together now. SWMBO needs the car at the weekend and it's going to rain tomorrow and Friday.