Open windows to help condensation problems in winter - good or bad advice?
Ok, basic gas laws.  Warm air can hold a lot more water than cold air.  (That's why it rains on windward side of mountains).  We define "relative" humidity as the percentage of water air can hold, at its current temperature. 

Fill a trash bag with outside air at 9 degree C 70% relative humidity.   Bring it indoors and let it warm to 25C.  The relative humidity goes down - and becomes so "dry" that it's uncomfortable, even though it's the same air and it didn't feel dry when you were outside.    Your indoor air is wetter than that, because of exhalation, bathing, cooking, and water penetration through floors and roof leaks. 

Take a bag of your wet 25C indoor air, outside - at 9C the relative humidity will go over 100% and water must condense on the surface of the bag. 

That is what is happening in your closed-off rooms.  Warm, wet household air is leaking into those rooms. 

So the advantage to opening a window is mixing in the dry air from outside.  However that is only competing with the internal leaks between the room and the rest of the house.  You'll need to open the window (or seal the room) enough to win.  Of course then, you will have water ingress whenever it rains, and that water will add to the problem. 

The American way is to run a dehumidifier, retrofit insulation, or just heat the house anyway.  But then, the American way is to pay 20 pence per litre for oil.