Martin Angove wrote:

Install a fan with a humidity sensor and ignore the "switched live"
input. Set the sensor correctly and it'll come on a few minutes after
you start to run a bath and go off when the room is sufficiently
aired.

I'm in the market for a bathroom extractor fan and wonder if its worth
paying a bit extra and getting a heat recovery unit. BES sell a baxi
ec25h @ =A3188.30(13687) that has a built in humidity sensor. A bit of
googling reveals there is another model EC25P which has a PIR to start
the fan.

Its probably one of those novelty factor devices that has an very long,
if ever payback period.