The experts in Locksmith in Erith suggest having a peep hole or a spy hole installed on the door. This provides even better security as one can check who the person is before even opening the door. With this, people can observe whether they know the person or not and decide if to open the door.
Locksmith Erith, 020 3868 4490, Fraser Road, Erith, United Kingdom, DA8 1QH, http://www.locksmith-erith.uk/