Discovery dust leaks

You'll probably find there's a little louvre gizmoin there which is supposed to shut automatically, but doesn't. I recall being told about it being a problem. Replace or seal permanently.

See ya dusty

I've removed all the trim from inside the back of mine completely. There is a rubber flap thingo on the right side which allows cabin air out but not dust in. The other thing to check is that all bolt holes throught the panels have either bolts or plastic plugs in them. Mine has no dust ingress at all, despite the addition of quite a few extra holes for extra equipment, wiring etc. Also check the boots over the wiring to the rear lights.

Take it back to LR immediately BP. It's obvious you've been done out of the standard holes to let the water in, to wash the dust out!
My Td5 lets a bit of dust in but luckily now it's supposedly Autumn/Winter it should get washed out OK.

Had the same problem on my Disco 1 & having removed the trim on the drivers side rear there were a couple of vents that are supposed to let cabin pressure out. The flap (valve) was missing completely on one so I sealed them with duct tape. No dust problems since or lack of oxygen in cab as it seems to find its way in/out elsewhere.
Cheers

G'day,
sealing of the flap located behind the trim on the RH side of the load area, is the trick(as has already been said).
I live on the cape and rarely see bitumen, not that it was a huge problem, but I did get a small amount of dust at times. The vents are very effective at letting air out to promote flow of fresh air thru the car. By sealing up these vents you will in effect be forcing any air that enters the car to exit via all the small leaks that form at various points around the car(or to state the obvious, you will be pressurising the cabin extremely well). Of course for this to work you have to have your windows up and blower on.
If you do this, suds your car up alot when you next wash it, you will be amased at how the thing will blow bubbles from around the door seals etc etc................well that is my experience anyway.