Well, it gets a bit technical, but negative pressure is part of it, and paying a little more attention to service entry holes (2mm gaps around the radiator pipes, and so on). I had used boards over the floor because of gaps between the (300 year old) floorboards and the absence of a ceiling below, but that wasn't enough. I should have put dust-sheets down instead, and thrown them out of the window when I'd finished. Back to negative pressure.....this is basically having an extractor running constantly sucking air out of the room to the outside, which means that air gets drawn in through all the little gaps which had released the dust into the rest of the house. With the air all moving into the room it should in theory prevent dust escaping into the rest of the house.
I've got two more rooms to do, and I've learned my lesson. I will be mounting a big old Expelair on a temporary board and fixing this in place at an open window, and I'll be going around the rooms with expanding foam and filling even the slightest gap.