It is definately road dust, it commes out of the door fittings (locks etc). These cars get wiped down almost daily. Toyota have now told me not to run the a/c on recirculate as that causes low pressure inside the vehicle which is causing the dust to be sucked in through the doors.
They now have a new problem on both of them. The door seals on the top half of the tailgate have worn through the paint on the bottom half where they join up.
Thanks for your reply anyway.
PS. I have attached some pics...sorry about the quality.