[Rob]

Black mould like that is normally condensation. You do need to work out where the moisture is coming from.

Is that ceiling against an external wall and to a roof? If so there could well be a lack of insulation pushed against the joint creating a "cold bridge". There may be physical building work needed to address it, and try to fix the joint. You also need to look at the amount of moisture you are trapping inside the home (simply breathing in a bedroom overnight creates a significant amount that's going to go somewhere), as well as looking at ventilation and temperature.

I'll attach an info leaflet on condensation I recently prepared, in a moment or two.