What is a suitable punishment for those who kill a policeman? For PC Andrew Harper’s widow, Lissie, the sentences given to her husband’s teenage killers – a maximum of 16 years – were woefully insufficient. Many agreed. And now Lissie Harper has started a petition calling for ‘Harper’s law’. Half a million people have signed up, demanding a change in the law to ensure those convicted of killing a police officer, firefighter, nurse, doctor, prison officer or paramedic are jailed for life.