Burst pipe or major leak

Do this

1. Turn off the mains water at the stopcock (see that entry).
2. Turn off the immersion/heating if the leak is from a hot system.
3. If water is near sockets, lights, or the consumer unit, switch off electrics at the consumer unit.
4. Open all taps to drain the system quickly and reduce pressure.
5. Catch/soak water; move valuables and electronics.
6. Call a plumber and notify insurance (contacts page).

Never touch electrics standing in water.