OK, thanks for all that...

But...woah!!! - "back pressure calculations" - OK, can anyone help me
by explaining this? (sorry for being ignorant)

Also, "Bear in mind when you are pumping air out of the house, fresh
air has to come in somewhere. "

If the fan is running (eg 20 mins after leave room) and all windows
and doors shut..can this damage the fan - ie does it have to work
harder??

Thanks