Been there done that. Get a dehumidifier. End of story. Do some research find a quiet one and put it on the landing somewhere. No need to place it in the room that has issues, air will find its way around your house. We had terrible damp and condensation, put a dehumidifier in and within a month it was sorted. When we first put it in, it ran flat out for about 2 weeks without cutting out and was shifting about 10 litres every 24 hrs. It now runs as it should and cuts in and out as and when it needs to. It now gathers about a litre every 36 hrs or so.