A teenager killed his "gentle" stepfather with a single punch after they clashed at their home, a court heard.

Douglas Herridge lashed out at 6ft 4in tall national squash champion Colin Payne with "catastrophic" effect.

The 54-year-old businessman instantly collapsed to the floor at his Dartford home and died soon afterwards from a devastating brain injury.

The tragedy was witnessed by Herridge's mother Ina, who told police Mr Payne "would not even harm a fly", Maidstone Crown Court was told.

Mr Payne had himself told officers he "couldn’t fight his way out of a paper bag" after an earlier incident with the teenager.

“Mr Payne was fit and healthy,” prosecutor Richard Hearden told the jury of seven men and five women.

“How did he die? He was killed by his 17-year-old, now 18-year-old, stepson, who punched him during the final of many recent arguments.

“That single punch was catastrophic and led rapidly to Colin’s death. It was witnessed at first-hand by the defendant’s mother and Colin’s partner.

“Douglas admits throwing the punch but says he acted in lawful self-defence.”

Herridge’s mother started giving evidence from the witness box without being screened from the dock where he sat.

She described before the lunch break how Mr Payne went slowly to the floor in the study after being struck on the left side of the face by her son’s left hand.

Mr Hearnden earlier told how Mrs Herridge’s parting and subsequent divorce from her husband David in 2011 caused “emotional turmoil” in Douglas, the younger of their two sons.

Herridge would become highly upset, angry and agitated.

There was an incident when he put his hands on his mother’s throat and others when he punched and kicked her and tried to push her down the stairs.

Mrs Herridge, originally from Finland, met divorced father-of-two Mr Payne, who ran three bathroom stores in Tunbridge Wells, Reigate and Brighton, in February 2014. He moved in with the family after about six months.

Mr Payne was fit and healthy and a keen sportsman, having been the British Over-50s squash champion and a professional player on the PSA World Tour.

Because of Herridge's behaviour it was decided the teenager should move out. He did so for about 18 months before returning in the summer of 2015.

He began studying at college but left after a short period. He then worked for a taxi firm followed by an air conditioning company.

After leaving there his behaviour deteriorated again. Mr Hearnden said it escalated and began to kick in on November 16 last year, three days before Mr Payne died.He told Mrs Herridge he was frightened by the behaviour.

“As far as Ina is concerned, she said Colin would not even harm a fly.

"Colin told police on that Wednesday before his death when he was an 11-year-old schoolboy was the last time he had a fight.

“He said he could not fight his way out of a paper bag. The police did not make any arrests and everything seemed to calm down. They left and the family got on with their lives - until the day of Colin’s death.”

“How did he (Colin) die? He was killed by his 17-year-old, now 18-year-old, stepson, who punched him during the final of many recent arguments" - prosecutor Richard Hearden

On Saturday, November 19, decorator Ziad Natour arrived at the house to do some work. Mrs Herridge was in the study on the ground floor studying for college.

Herridge went to see her and asked for money. She refused, pointing out his allowance had been stopped and reminded him how bad his behaviour had been.

He became tearful and agitated and swore and flapped his arms about. He was getting wound up and aggressive.

Mr Payne heard the commotion and entered the room. He told the teenager: “What’s going on? You need to stop swearing.”

But it did no good as Herridge did not calm down, said Mr Hearnden. He tried to stop Mr Payne entering the room.

He managed to get in but Herridge continued shouting and calling Mr Payne names, including saying he was “a pussy”.

“This appears to have made Colin annoyed and agitated,” said the prosecutor. “Both took hold of each other. It looked like both held each others shoulders.

“Colin was shouting ‘Come on, come on then’. Ina got between them, shouting to stop it. But things moved too quickly.

“A punch was thrown. Using his left fist, Douglas punched Colin to the side of his head. It landed on his temple, it seems. The result of that one punch was catastrophic.

“Colin slowly collapsed and fell to the floor. His nose began to bleed instantly. Ina must have feared the very worst. She shouted: ‘Colin, Colin, look at me. Stay with me.’

Douglas walked out of the room. Ina called to the decorator and he came to assist. Colin, meanwhile, was dying. His lips had turned blue.

“His breathing had turned into what sounded like snoring,” said Mr Hearnden. “He had lost consciousness, in fact.”

Mrs Herridge said she was studying at home for a masters degree in business administration on November 19 when Douglas asked her for money.

She refused and he became emotional and called her names, swearing at her.

Mr Payne entered the room and asked what was going on. There were raised voices from him and Douglas.

“It just happened so quickly,” said Mrs Herridge. “As soon as they started to shout I got hold of my phone, thinking I might have to call for help.

“I went between them. I said: ‘Can you stop.’ They were holding on to each other with their arms. I couldn’t see where their hands were.

“It was like they were in a different world. The didn’t hear me. I left the room to call for Zi (decorator Ziad Natour). I said: ‘Can you come down and help me? Call for help.’

“I went back to the room. They were in the same position. Douglas said: ‘You are a pussy.’ Colin said: ‘Come on then.’ Then I saw one punch. Douglas punched Colin.

“I think it was his left hand. He is right-handed. The punch landed on the side of his face. At this point, Colin’s back was against the wall.

“Douglas left the room. Colin put his hands around his head. He started going down very slowly. I was shouting to Zi upstairs. I said: ‘I need your help now. Call the police, call an ambulance.

“He (Mr Payne) was slowly going down. I thought he must have a level of consciousness. I could hear him start snoring. He went to the ground.

“He was sitting upright when I heard snoring but slumped forward. Zi came down. I was checking Colin’s breathing and pulse.

“I had been a nurse. I did CPR. I got instructions from the operator. Paramedics came and took over.”

Mrs Herridge said she had never seen Mr Payne get into any fights before, adding he was not a violent person.

Herridge, of Sackville Road, Dartford, denies murder and an alternative charge of manslaughter.

The trial continues.