Two amateur boxers pleaded guilty yesterday to killing a businessman who tried to stop them beating up a tramp in a city centre street.

Tom Cowles, 22, and his brother Ben, 21, were part of a gang of young men who were attacking the tramp in the early hours. When Frank McGarahan, 45, stepped in to help their victim, they turned on him.

They held him in a headlock, punched him in the head several times and knocked him to the floor.

Closed=circuit television footage of the killing was played at Norwich Crown Court yesterday. It showed Mr McGarahan staggering from the blows before collapsing and lying motionless.

The Cowles brothers, both members of Norwich Lads Amateur Boxing Club, had denied murder. The prosecution accepted their pleas of guilty to manslaughter.

Mr McGarahan was chief operating officer of Barclays Wealth. He had some of the world’s richest people among his clients and managed assets worth £133 billion. On the night of the attack he had been celebrating a christening in Norwich with members of his family.

The CCTV footage was played in court yesterday. It showed Mr McGarahan’s brother, Kevin, being punched to the floor and lying motionless. Mr McGarahan’s cousin, Sean Ryan, was also punched to the floor as he tried to help.

Mark Dennis, QC, for the prosecution, said that Frank McGarahan, his brother and cousin had gone to speak to the group. They asked what was going on and footage showed them approach with “non-aggressive intent”. They were met with abuse and told to “f*** off or you will get some”.

He said that one of the group of youths had tried to justify the attack on the tramp by saying: “You don’t know what he has done. He has got what he deserved.”

Mr Dennis said that Kevin McGarahan had then called the group of youths cowards.

Ben Cowles, a pipe fitter, “took a run” at Frank McGarahan and punched him in the head, Mr Dennis said. Cowles then attacked Kevin before punching Frank again. Mr Dennis said: “It was a blow which was followed a few seconds later by Frank McGarahan’s sudden and dramatic collapse.”

Frank McGarahan, of Much Hadham, Hertfordshire, died the day after the attack last September. He was the father of two girls.

At the time his family issued a statement, describing him as a fair-minded man who could not stand by as someone was being attacked. “He tried to break it up . . . he paid the ultimate price for trying to be a good citizen.”

The judge, Mr Justice Saunders, told the court that the prosecution’s decision to accept a guilty plea to manslaughter was “sensible and appropriate”.

The Crown Prosecution Service said that there was not enough evidence to convince a jury that the Cowles brothers had intended to kill or cause grievous bodily harm.

Sentencing was adjourned until later this month.

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