Frank McGarahan's trip to Norwich had a celebratory motive. Having travelled 80 miles from his home in the quiet Hertfordshire village of Much Hadham, he was in East Anglia for the baptism of his niece Thea, the first child of his youngest brother.

So it was no surprise he was in a good mood. And, following a family meal on the eve of the christening, Mr McGarahan, his brother Kevin and their cousin Sean Ryan decided to carry on the festivities. After drinking at various city centre bars the three called it a night and made their way to the taxi rank on Guildhall Hill.

It was at about 3am on Sunday 28 September 2008, while waiting for their carriage, that Mr McGarahan, whose job as a senior executive for Barclays Bank had made him a millionaire, spotted something that alarmed him: a gang of up to 12 young men attacking a homeless man.

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Rather than ignore it, Mr McGarahan, 45, walked 20 yards to remonstrate with the group. His concern cost him his life. When his back was turned Ben and Tom Cowles, two brothers who had trained as amateur boxers, attacked him.

Ben, 21, had "taken a run" and punched Frank in the back of the head before turning his attention to Kevin, beating him to the floor.

Tom, 22, then took over assaulting Frank, putting him in a headlock and punching him at least 10 times. Their friend, Daniel Moy, 21, punched Mr Ryan to the floor. Then, after a further blow to the skull from Ben, Frank collapsed "suddenly and dramatically". He never regained consciousness and a post-mortem showed he died of bleeding to the brain. Doctors noted that more than 20 marks and abrasions had been inflicted on the father-of-two, and he had suffered serious head and neck injuries.

As he lay dying, the attackers "turned on their heels and fled", dodging the revellers still milling around the town square as they ran. The following morning Father James Walsh, dean of Norwich's Roman Catholic cathedral, received a phone call informing him that the 11am baptism of baby Thea would need to be cancelled. Instead, the McGarahans ended up planning for a funeral.

Two days after the attack on 30 September, the Cowles brothers, from Norwich, handed themselves in to Norfolk police. Both admitted taking part in the attack and said they had reacted after being called "cowards". Yesterday, however, Tom, a builder, and Ben, a pipefitter, pleaded guilty to the lesser charge of manslaughter and to affray, relating to the attack on Robertas Sinkevicius, a Lithuanian "down and out". They will be sentenced on 26 June. Moy, a carpenter, had already admitted assaulting Mr Ryan. He was fined £50 and ordered to carry out 100 hours of unpaid work.

In court, prosecutor Mark Dennis QC said that after the three victims had witnessed the attack on Mr Sinkevicius they intervened because they feared he would be badly injured. "It was clear that someone needed to step in to prevent someone being seriously hurt. In the circumstances they were brave to intervene," Mr Dennis added.

Had the trial gone ahead, much of the police evidence would have been from what was captured on CCTV cameras. Speaking about the attempt by the McGarahans and Mr Ryan to intervene in Mr Sinkevicius's attack, a police source said: "Their body language clearly shows that they were not being aggressive, they were not threatening, they were simply trying to help someone who was being attacked and hoping their presence would calm the situation.

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"They were attacked without provocation and warning, and what happened shows the dangers of punching people in the head. They behaved as good citizens should, by going to help without being aggressive and paid a terrible price."

They would also likely have relied on evidence from Darryl Lawton, a doorman from the nearby Spearmint Rhino gentleman's club. After hearing the attack on Mr Sinkevicius, he too went to intervene and ended up witnessing the death of Mr McGarahan.

At the time he said: "There were two men on the ground being attacked. It was fists and feet. It was extreme violence." Mr Lawton said he had caught one of the attackers, putting him in a headlock, but had to let go after a volley of kicks from other members of the gang broke his jaw. The suspects escaped but left two men – Mr Sinkevicius and Mr McGarahan – lying in the street.

Mr Lawton, a trained first-aider, said: "One of the men wasn't moving and was quiet. He wasn't breathing. The other man's face was covered in blood, but his hand was moving. I put the man who was quiet into the recovery position and started working on him, giving him mouth to mouth. But there was no response and I knew there was a problem."

Mr McGarahan, who was in charge of 7,700 members of staff and managed assets worth £133bn as the chief operations officer for Barclays Wealth, died at Addenbrooke's hospital in Cambridge the following day.

His older brother Tony, who was in court, said shortly after the tragedy that the family would "never truly recover". "He did what was natural to him – try and help someone – because they needed help," he said. "We will always be proud of Frank and we will never ever forget him. He was a fantastic father to his two young beautiful daughters, a loving husband to his wife [Alison], a big-hearted son to his mum and dad and a brilliant brother and uncle. He was simply a great family man."

The court also heard that Kevin McGarahan had told police the three men had approached with "caution" and had shown no "aggressive intent", but were attacked without provocation. He added: "There is a sense of devastation over what has happened to my brother Frank. His death destroyed us and ripped our family apart. He has left behind a wife and two young children who will never know their father. All we were trying to do was help a man who was in trouble."