Had this problem on mine a few weeks back. First of all both pollen filters were knackered, also the fine gauze that covers the air intake just beneath the windscreen had perished. When in rained, the water simply accumulated in the blower intake duct in the footwell and seeped into the car.
Remove the glovebox and the panel beneath, undo the wiring plug and loosen off the cross-head screws that secure the blower to the housing. remove it, clean away any debris and mop up the water. Refit in reverse.
As regards the mesh, there is a LR replacement but it looks a pain to fit so I simply removed the screws that secure the bottom edge of the plastic screen surround by the wipers, cut some black aquarium sponge the size and fiddled about until it was in position to cover the air intake slots. No more puddles since.
The pollen filters are easy, but make sure they seat properly when you replace them and that the plastic cover forms a perfect seal othertwise you will be straight back to square one. many people like to run some transparent sealant around the covers when they are done just to make sure.
More details on these fixes on rangerovers.net
Hope this helps