Keep clear of hazards
Keep people away from sagging ceilings and water near electrical fittings. Do not touch wet switches, sockets or appliances. If there is immediate danger, seek emergency help. Only isolate the water using a known accessible valve when it is safe to do so.
Think about access above
For a flat, inform the relevant occupier, landlord or managing agent promptly. Access to the home above or a communal riser may be needed. The stain below does not by itself prove which fixture or property caused the leak.
Keep a simple record
From a safe position, photograph the visible damage and note whether the water appears after bathing, heating, rain or at all times. Record any action taken to stop the flow. Share this timeline and access contacts before an appointment so the investigation can focus on the right systems.