Many thanks for the responses. Yes the vents do conduct cold air
directly from outside into the wardrobe space. I've been told that I
need these left open for air flow - although am I right in thinking
that the air flow must be warm, not cold? Why would someone have been
so stupid as to vent cold air directly into the wardrobe?

The wardrobe is a built-in type with sliding doors. Would I be best to
fit polystyrene ceiling tiles to the insides of the doors or the wall?

Thanks