Roof repair?

I have dampness appearing, following rain, on the inside of a bedroom window. The dampness is low down on the small side wall, coming out from the window frame, just above the window-sill. Outside is well maintained, with good seals and wall covering. The loft space is dry. I think the water is coming down through the wall cavity and landing on the lower window-sill support inside the cavity. Is this likely? What type of repair would be needed? Would this be a roofer or general builder? Roof is tiled.