As part of Miss Ham's bathroom refurb, I replaced the extract fan.
The original xpelaire had two way switch 3&earth cable, of the three wires, one (appeared to be) permanently live, the second appeared to be switched in parallel to the light so the fan went on when the light was switched on and carried on running until the internal humidistat switched off.
Replaced with a pull switch device, two wires, connected to the perm live and neutral.
All seems well, except that when you switch on the fan the (now LED) light turns on, too.
Can't get my head around the logic of that.