The definition of a night latch is spring lock that can be opened from the outside with a key only and the inside by turning either a knob or short handle. This type of lock never needs to be locked with a key as just pulling the door closed will engage the latch into the strike plate. This type of lock can be found in all different shapes and sizes but the main thing is that they all work the same way. The basic models purely work that way but you can get more complex models that are able to be double locked from the inside and outside. You would normally double lock these from the inside by either pushing the snib (button) up or down depending on what side of the door the lock is fixed to. The snib is seen on the picture below just to the left of the handle. Double locking from the outside is achieved by closing the door, inserting the key into the cylinder and either turning it a full turn to the left or the right again depending on what side of the door the lock is fitted to.
You are also able to get upgraded versions of this style of lock that are of a British Standard level that double lock as soon as the door is closed but locksmith Reading and locksmith Bracknell don't feel that is necessary if a 5 lever British Standard deadlock has been fitted to the same door.
These locks are not normally used as a security device but purely as a means of keeping the door closed but us at locksmith Reading & locksmith Bracknell believe it's fine to just use this lock as long as someone is inside the house and not sleeping. Once either the last person has left the house or gone upstairs to bed then I definitely recommend locking the door with a mortice lock pictured below which should have a British Standard kite mark on the faceplate.