1a. For leaks originating within your flat: Turn off your water supply. Please familiarise yourself and test whether you are able to turn off the water or not.
1b. For leaks originating outside of your flat: Make contact with the owner of the flat you believe the leak my be originating from.
2. Mitigate the damage as best you can using buckets, containers and towels.
3. Contact the owners of other flats you believe may be affected, such as those above or below you.
4. Notify your letting agent or landlord of the damage.
5. Notify your insurance company of the damage.
6. Affect any repairs as necessary.