Seems alittle bit ad hoc though. I have no fan, I open a window!
Brian

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"Harry Bloomfield" wrote in message
news
on 13/05/2018, me supposed :
And as the regs require this ventilation when the shower is in use surely
there's no point operating the extractor from a light switch because then
it doesn't have to be used.

Fit a PIR operated fan. That would cover the bath, or shower, or toilet
being used in a room. Mine includes both a built in PIR and a humidity
sensor. It comes on if you walk in the door and it continues to run after
the bath or shower has been used, until the excess moisture has been
dispelled to the outdoors.