Nicholas van Hoogstraten and Mohammed Raja were both driven players in the "murky" world of property dealing.
Hoogstraten felt Raja had shown him disrespect by challenging him.
The elderly Mr Raja was taking civil court proceedings against Hoogstraten, alleging fraud.
If successful, the proceedings could have led to criminal charges against Hoogstraten - and the threat of prison.
But Mr Raja feared Hoogstraten and was constantly on his guard after launching the action. He also warned the rest of his family to be on their guard.
His son, Amjad, said: "He had heard Hoogstraten was going to take some sort of revenge or action against him because he wanted to stop the litigation going on."
Amjad last saw his father alive the weekend before he was stabbed and gunned down in the hall of his home in Sutton on July 2 1999.
Two men, Robert Knapp and David Croke, posing as gardeners, had knocked at his door.
They had been hired by Hoogstraten and had a sawn-off shotgun concealed in a fertiliser bag.
Hoogstraten had paid Knapp about £7,000.
In court, the tycoon described the fee as "laughable" and claimed it would be the first time in history that alleged killers "had been paid by stage payments".
But Hoogstraten had a reputation for meanness - police found teabags drying on his draining board when they searched his premises.
He had used Knapp as an enforcer over the years and trusted him implicitly.
Knapp had met Croke in jail. Croke had recently had been released from a 20-year sentence for brutal armed robberies.
Amjad said he had once met Hoogstraten while discussing one of his father's properties with another business associate, Michaal Hamdan. Hamdan and Raja were also in dispute.
"I told Michaal, 'My father is a thorn in your butt', and he laughed.
"Mr Hoogstraten stepped forward and said 'Those thorns - we pick them out, one by one and we break them'. He was in a serious mood."
Hoogstraten said he could not recall when he first heard of Raja's death, telling the jury: "It was not the highlight of my life.
"I was very sorry and concerned about it, but I did not for a minute think that anyone would be coming round to see me in connection with it."
Mr Hamdan, he suggested, had a motive for wanting Mr Raja dead.
But Mr Raja's grandson, who was a witness at his death, told the court that Mr Raja had named Hoogstraten's men as his murderers.
The grandson, Rizvan, flew from Pakistan, where he was studying, to testify.
He had been talking to his brother upstairs when they heard a loud noise and loud bang.
He said: "We heard voices as if someone was screaming at each other. I went downstairs with my brother behind me.
"I saw my grandfather standing in the hallway and two gentlemen. One of them was reloading a sawn-off shotgun.
"My grandfather was looking at the men. As we came down he turned and looked at us. There were drops of blood over his shirt and he was holding his chest or stomach.
"He pleaded in Punjabi. He said 'They are Hoogstraten's men and they have hit me'. He repeated that twice. Then he called on his mother, 'Mum, they have hit me'. She passed away in 1985."
Rizvan saw one of the men was bending down, loading the gun.
"It was open. He was putting a cartridge into the barrel. He was wearing a hat similar to one worn by Jack Russell, the English wicketkeeper. It was covering his eyebrows."
The other man was holding a garden fork and wearing a moustache, Rizvan said.
He added: "There was something odd about it. It did not seem right on his face."
Rizvan said he went into the kitchen to try to call the police, but got an answering machine telling him he was waiting in a queue.
He said: "I threw the telephone down. I was facing the hallway and saw the gunman point the gun towards me.
"I slammed the kitchen door. I thought that at any moment the bullet would come through the door. I threw myself on the floor.
"Then I heard my grandfather again as if he was in pain.
"He was screaming. The noise was coming from the TV room. I went to the doorway and saw my grandfather was holding a black kitchen knife in one hand.
"The gunman had aimed the gun towards him. He had the gun on his shoulder. He was aiming towards the chest and neck area.
"Then I just heard a loud bang and it hit my grandfather. He fell down and the gunman ran away."
Rizvan said he checked Mr Raja's pulse and felt his chest but could not tell whether his heart was beating.
He jumped over to another telephone. The receiver was still off in the kitchen and he found he was still in the queue.
He said: "I put a towel over my grandfather and then jumped over the fence to the neighbour next door to ask him to call the police."
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