A convicted murderer who attacked a prison officer at HMP Whitemoor in March has been sentenced. Christopher Walters, 35, launched the unprovoked attack on January 16 last year when he punched the prison officer in the face twice, knocking him unconscious. The victim was taken to hospital for treatment and was discharged the same day. In 2013 Walters was convicted of murdering a man in London in May the previous year. Walters, previously of Neasden in London, initially denied assault causing actual bodily harm but pleaded guilty during a trial at Cambridge Crown Court yesterday. At the same he…