It is unlikely that your mouldy curtains, or indeed the majority of
symtoms you are suffering in the room are the result of anything other
than condensation.

Which elevation is the extension on - North, North East? or is it in a
particularly windy environment?

Also, being built in the fifties, It will have dreadful u values in the
walls and floor which aren't going to help.

Heat it better, ventilate it better (does it have PVCu double
glazing?), generally follow all good guidelines to combat condensation
and your mould will disappear.

If you do indeed have wet joists this could be caused by a lack of damp
proofing on the sleeper wall, as SS suggests. Howver, if this is so,
it's an error on the builder's part and, to be honest, if you've had
wet joists sincer 1950, you're very lucky you haven't got any kind of
rot. You should check on top of the sleeper walls for a black
substance between the brickwork and the timber, probably plastic but
maybe bituminous which is acting as a dpc. If you can't see it, the
sleeper walls may be built directly off the oversite and they've put a
dpm beneath that.

If you really haven't got any form of dpc then you're going to have to
slightly jack up east joist (quarter of an inch or so) in the vicinity
of the sleeper wall and slip a piece of dpc between the wall and the
joist - no big deal. To be honest, this is unlikely, it's more likely
to be sub-floor condensation.

If you can hire (beg, borrow,steal) an electronic damp meter, your
joists should have a mosture content of less than 22%. Ideally it
should be less than 20% but sub floors are never ideal places and
they're usually on the high side. Don't use it on the walls, you'll
frighten yourself with the levels of condensation you obviously have

If you want any more info on condensation, email me directly and I'll
give you the url of my website.

HTH

Xav

Have alook and post back