I own a bottom flat of a villa divided into two flats. The owners of the upper flat have converted the loft area and have always maintained the roof now they are demanding half the costs of new roof?

You could give them part of the cost, but they have to put a roof on for their benefit. Unless there is a contract in place, I don't know that they can force you to pay. You do share the roof and you do get benefit from it. If a car hits the side of the villa and only the first floor is damaged, are they going to help pay for it even though it is holding their section up? I doubt it. If they are asking for payment after the fact, you were not asked about type of roofing, who was chosen to do the work, or if you felt it was needed.