On Tue, 23 Feb 2010 05:13:51 -0800 (PST), Bear
wrote:

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?

You need airflow at room temperature, otherwise warm air (which can
hold more moisture than cold air) enters through gaps in the doors or
when you open the doors and the moisture in the warm air condenses
onto the cold clothes and wall.

Why would someone have been
so stupid as to vent cold air directly into the wardrobe?

They were probably already there when the wardrobe was built and
whoever was building it didn't close them off. Did the room
originally have a fireplace or gas fire in it?

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?

The wall. The wall will be the coldest part even after blocking the
air vents so to minimise condensation you need to insulate it so the
surface temperature of the wardrobe side of the insulation layer is at
room temperature. You don't want any insulation on the doors as you
want heat from the room to get in.