Basically, the rooms are small and I only used a large desk fan, that I ducted out of the window.I have been looking at the ducted fans - I was thinking of a 12" 3600m3/hr. which seems pretty powerful. what did you use?
But as @opps has said, there are methods builders can use to avoid much of the dust being created in the first place. Don't get carried away with extraction fans, when a false ceiling, or a thorough vacuum from above, will get rid of a century's worth of coal dust from the attic first!
It's better to ge rid of it at source, than to rely on extraction when it's already permeated every orifice!