Damp mould can be caused my many things - no ventilation in the bathroom when you shower, for example.
Rising damp, too.
You need to get into the loft space with a torch and have a good feel - can you feel wet or damp? Is there a musty smell?
Go up during strong daylight - can you see daylight through the tiles?
Is the mould under a water pipe? Under the bath, perhaps?
Don't assume you have a leaky roof. Damp can be caused by many different things.
You need to be sure it's not caused by condensation, leaking windows, gables, dripping pipes....
Take a deep breath and play detective. :)