Flooded flat: what to do
The order of actions after a leak: shut off the water and power, call the emergency service, record the damage, get the report, notify insurers and dry out.

[ ] Shut off the water
    Your own stopcock first; the riser through the emergency service. You need to know where it is before, not during.
[ ] Cut the power to the flooded areas
    Water and electricity together are dangerous. Switch it off at the consumer unit rather than unplugging with wet hands.
[ ] Move devices, documents and valuables
    Out of the affected area and off the floor. Rescuing is easier than restoring, and you have minutes rather than hours.
[ ] Call the emergency service and log the reference
    The time of the call, the reference number, what you were told. It is the first document and it fixes the timing.
[ ] Tell the neighbours below
    They see the consequences before you do, and their flat is part of the damage. A calm warning saves weeks of argument.
[ ] Find the source of the leak
    From above, from your flat, from the riser or from an appliance. If it is from above and nobody is home, that is the emergency service's job.
[ ] Photograph and film everything, with the date
    Ceiling, walls, floor, furniture, appliances, stains. In a week the traces are gone and the money conversation is not.
[ ] Get the formal damage report drawn up
    The responsible organisation records the damage. Insist that everything is listed, including what appears later: damp, swelling, marks.
[ ] Notify the insurer within the deadline
    If you have property or liability cover. Notification deadlines are usually short and stated in the policy.
[ ] Talk to the neighbours in facts, not accusations
    Most floods are accidental. A calm conversation ends in compensation more often than months of correspondence.
[ ] Do not sign off an amount immediately
    Some damage appears weeks later: swelling, mould, peeling. Assess the damage after drying.
[ ] Start drying immediately
    Ventilation, a dehumidifier, removing soaked finishes. Mould starts within days, and that is a different job.
[ ] Have the electrics checked before switching on
    Flooded wiring and appliances only go back on after a professional check. That is safety, not electronics.
[ ] Note what would prevent a repeat
    Know where the valves are, replace hoses and connections on schedule, fit leak protection, do not run appliances unattended.

Checklist from Unload: https://unloads.me/c/flat-flood