[MarkS]

The gearbox reverse switch and reverse lights have been tested and works consistently.

I have observed that the long tone only sounds the FIRST time the gearbox is put into reverse, and will not sound again, irrespective of the number of times the gearbox is put into reverse (from park or neutral), until the ignition is switched off fully.
Once the ignition has been switched off then back on (to position 2), the tone will sound once again (and first time only), irrespective of the number of times the gearbox is put into reverse.

One of the sounders was slightly corroded, the paint was scraped off. This had no effect on the non-working of the parking aid.

The Haynes manual 4292 states that it is important for the correct operation of the system that the sensors are aligned correctly - the retaining clips are offset to ensure this.
What should the alignment be on a tourer ? Can somebody supply a photo of all 4 sensors in the bumper please ?