The problem in as stated above water, passing into the plenum and depending on conditions water may travel into the heater duct entrance, the duct does rise up slightly but if water gets in it’s all down hill then, there’s opening from the plenum each side through the inner wings but still some water can remain laying in the base of the plenum.
I dont know what they mean with the numbers but LJ is model type (4th & 5th Characters) in the VIN for "Discovery" while for "Discovery Series 2" is LT and if you know well how TSBs are it's clearely visible from it's number 76/37/94/EN that it was issued in 1994 so it implies Discoveries built untill '94 i suppose.
Maybe it's just me, but I think I'd prefer to see where the water is going, rather than it dissappearing in to a hole, which as you say, should allow it to drain out somewhere else as long as the route is clear.
I had exactly that in my old D2. (In fact I thought you had nicked the photo I posted on here a while ago) My head lining was getting soaked above the windscreen. Sealed it up and the soaking stopped...
