After a bit of experimenting around yesterday I finally found a location for the sensor that restricts enough of the available light to make it actuate the headlights at the right level of light outside. It's still very discrete, especially when the door is closed and a bonus is that there is no routing of the wire under the windscreen pillar trim to mess around with.
Like previous posters I lost two of the steel spring clips that attach to the plastic dash end panel but managed to fish them out with no trouble.
While driving around yesterday the headlights were coming on for me at sensible outside light levels so I'll see how things go over the next week and decide if any further positioning is required.
One additional bonus (IMHO) is that the dashboard illumination is now permanently on, with the switch in Auto. A downside is that the sat nav display is now in dark mode however I've changed the setting so it's permanently in daytime mode, day and night.
One other thing I've noticed now is that the illuminated headlight symbol does not appear when the headlights are on.