A teenager accused of killing his “gentle” stepfather with a single punch during a row over pocket money is waiting to hear his fate after the jury retired to consider a verdict.

Douglas Herridge has denied the manslaughter of 6ft 4in tall former national squash champion Colin Payne, claiming he acted in self-defence.

Maidstone Crown Court has heard the 54-year-old businessman collapsed to the floor with “catastrophic” effect at his Dartford home on November 19 last year 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”.

The stepfather, who represented England at squash and won the over-50s British title, had himself told officers after an earlier incident at the house in Sackville Road with the teenager he “couldn’t fight his way out of a paper bag”.

“Mr Payne was fit and healthy,” prosecutor Richard Hearden told the jury of seven men and five women. “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, a risk assessor with the NHS, gave evidence from the witness box without being screened from the dock.

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

Mr Hearnden told how Mrs Herridge’s parting and subsequent divorce from her husband David in 2011 caused tremendous “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.

“We say he was not acting in self-defence,” he said.

Herridge, who has been bailed to his father’s address in West Malling, claimed in evidence Mr Payne had his hands around his throat when he lashed out at him.

He said he threw one punch that missed and then threw a second which caught him on the side of the face.

Asked why he hit out, he replied: “Well, he had hold of me around the neck and all I wanted to do was get him off me.

“When I struck the second blow I was scared. I didn’t know what was going to happen. I had to defend myself. I remember him going slowly towards the ground.”

Herridge, who said he liked Mr Payne, added he could not believe it when told he had died.