I had this problem last winter, cleaned off using strong bleach solution around March time, came back around end of November.
The damp problem seems to be limited to the bell cast area of ceiling, you can see damp free areas where wooden joists attach to inner ceiling. I have been told I have insulation in my loft but have not looked as the loft hatch is tiny, and I am not lol.
I am not sure if the bell casts are insulated though, as I have been told by a builder that came to price up a floor said that they would not be as the house is old (thick stone outer walls)
I have double glazed windows, although they do not have any trickle vents. they can lock slightly ajar.
any ideas as cause to damp/mould and any remidies I could try.
Could the shower be to blame? window is opened most time it is used, cant see this though as small bedroom directly opposite has no damp issues only rooms to left of bathroom and hall roof outside to right.