Keep the source question open
Water can run along timber, pipes and penetrations before it reaches the ceiling. The darkest point is not necessarily directly beneath the leak.
Arrange useful access
Identify the spaces and fittings above, including other flats. Coordinate authorised entry so relevant checks can be made.
Treat immediate hazards first
Keep clear of a sagging ceiling and water affecting electrics. Seek appropriate urgent help before routine investigation. The draft form is not an emergency contact route.
Explore the relevant service
General information for planning an investigation. The property, instruction and findings determine the appropriate work.