could be a few different things that is causing your damp issue without looking its impossible to say what,could be a broken tiles torn under felt chimney,condensation best bet is post the job on site and get a local expert to look at the cause and cost of repair.
good luck Alex

Hi yes, it could either be a leak caused by many problems or just a small problem I.e a slipped tile or slate. If you are saying mould I would put my money on ventilation. Your roof has to breathe or this is what occurs. Does your roof have vents

Damp in roof void could be a number of different reasons
Mainly a breach water ingress
Broken tile or tiles
Ridge tile or pointing to ridge tiles
Lead flashing to chimney or party fire walls
Poor ventilation to roof void
Next door roof could be the cause
Various different problems
Hope this helps
Nick

Its hard to say without a inspection but quite often the loft could be sweating due to no ventilation, in a house the heat rises and cant escape ,I would check to see if you have any air vents installed and if not get some and this should sort out the problem.
Kind regards Spencer Bartley

Mold in the loft area in my experience is sometimes caused by lack of air flow the roof has to have air flow like under your ground timber floors if you block the vents it can cause this problem or not enough air flow can do the same if the roof has old under felt with no vents vent can be fitted ,sometimes when people have there loft insulated they push the insulation down to the eaves which can block eave ventilation it's just easy fix just pull back the insulation from the eaves hope this information helps Eric