Lots of vids on this condition on u tube, it's no a specific disco issue.
Anyway, it was a know issue that may happen or maybe not, a few head lamps were replaced under warranty by Land Rover because of it.
I even drove a new facelift demo vehicle which had a problem in one of the lamps and that was after Land Rover starting fitted a modified vent.
If you search u may find that some owners have removed the rubber vent compleatly which they say is better.