How can I stop my cystern getting condensation on the outside? ...
grill your toilet/bathroom
condensation forms when hot water fog meets something cold and reform into water.
You could try adding together ventilation into your toilet, later there will not be so much hot marine vapour within the air. Or you could warmth your cistern, but that is pretty daft.
Answers: Annoying isn't it? The only entry I can think of is a de-humidifier within the room. We have equal problem.