The whole thing seems to revolve around the fact that pressed against the windscreen is a **** off camera which looks like something out of a 1960s jet fighter. It's almost all heatsink.
Apparently this car (Citroen) needs the calibration done "on the road" - it can't be done in a workshop.
I would have quizzed the technician, but the car was in storage while we had a replacement.
I'm trying to understand how the system can be so sensitive that the slightest disturbance to the camera requires a recalibration (as the leaflet left behind advises).