gordon robson , 26 , hit father john potts , 45 , with a single punch to the back of the head .
robson , who has a history of violent offending , was jailed for just three and a half years for the attack outside the ne38 sports bar in washington , tyne and wear , in november last year .
the court was told that robson had been drinking at the bar after being a pallbearer at his grandfather 's funeral before getting into an argument with a friend of mr potts .