Based on previous discussions here, I finally got around to having the
extractor fan set up in the bathroom.

* S&P 250 m3/h quiet fan mounted above the ceiling
* humidistat + timer control on the wall close to the ceiling
* flexible ducting from a grate close to the centre of the ceiling to
the fan, & from the fan to the grate in the outside end wall (above
the ceiling)

The fan is audible, but by no means noisy. I am amazed by how
effectively it removes condensation in the bathroom (I was going to
try this & then consider an illicit dehumidifier arrangement) ---
within 24 hours of firing it up, even with showers in that time, all
the condensation cleared off the ceramic toilet tank, & it has stayed
clear.

The window is still locked "vented" (about 10 mm open) for the summer,
so the fan is mostly replacing bathroom air with outside air. Can I
expect such good results during the winter too, when it's sucking
"indoor" air under the door?

(It's a small bathroom with a radiator. The door has a spring-closer.
The ceiling is now insulated.)