Tim Mitchell wrote in message ...

Sounds more like condensation from the inside room air due to the wall being colder at that point.

Thanks for all your replies.
Just to make it clear, the air vents are plasterboarded over, so
wouldn't vent smoke directly from the room - although I've no idea why
else they would be there!

Tim's suggestion makes sense, although the damp is on the "outside"
side of the plasterboard: wouldn't the condensation form
preferentially on the stone "tunnel" that goes to the vent, rather
than on the back of the (nominally) warm plasterboard?

Either way, blocking it up for a while seems a good idea. Is there a
simple way I can measure the damp level, to assess if blocking the
vent off has worked?
Should I open up a grille in the bedroom, to ventilate the void behind
the plasterboard?

Thanks for your help,
Nathan