If it is a drain then a probe (suitable piece of coat hanger wire) would poke thru to something. Not sure, but this might be that same Rover in all the pictures in the D2 RAVE manual.... kept in a jar and taken out just for pictures.

 

Yep it's a drain hole. How water can get up there shouldn't be to hard for any off roader to find out...

The location pegs are on the lower intake manifold itself.

 

Start the engine and then put your finger on that hole if you can.
Don't do it when the engine is hot.
See if it is sucking in air.

That may be in the middle and where the gasket is.

I don't remember a hole in my plenum

But what do I know.
This was 20 months ago now for me

Flickr: Landroverdude2's Photostream