Roverman, if the water is coming in through his cabin air filter ingress--- underneath the front windshield plenum, the solution isn't to block the ingress off.
You don't want water pooling under there--- as you have the windshield wiper motor electrical wiring/connector two feet away.
I would check to see if it's his A-pillar trim that has one of its clips loose? To pull off the A-pillar trim, all you need to do is grab it firmly on each side, and pull outwards. Each A-pillar has 5-clips. It uses the same clips that your windshield plenum uses.
Land Rover has a "kit" with 10 clips & two velcro pieces (enough for both A-pillars). The part number is (DHB500400 - KIT, REPAIR KIT, 'FITS TO "A" PILLAR'). It's about $12.
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If the aforementioned isn't the cause, then it's the sunroof drainage tubes.
There's a write-up over at Disco3; here's the link: disco3[dot]co[dot]uk/forum/topic22757.html