What can cause condensation in a loft and how can I cure it?

lack of air circulation / ventilation, without looking in your loft cannot tell you how to solve it. post it in the roofing section of the site and get someone round to fix it

Lack of ventilation. Check you have minimum 270mm insulation, if you have eaves vents make sure they are clear and not blocked with insulation, you need a good air circulation. If you dont have any vents, get them fitted, either by soffit vents or tile/slate vents

a lack of ventilation is the most common cause, a breathable felt is the best answer but exspensive, may get away with tile vents, consult a local roofer for the best option