Dampness and water streaks on ceiling of top-floor flat. What could it be a sign of?

I live on the top (second) floor of a converted Edwardian terraced house in London.

I have recently noticed dampness and – when it is raining heavily – water streaks along the corner of one side my bathroom ceiling. What could all this be a sign of?

As joint freeholder (with the owners of the flats on the other floors), the entire building is covered by quite comprehensive building insurance. If I am an approach our insurance provider, what should I say to them?