He was the first member of the Chinese organised crime gang whose 'menacing reach' extended into extortion, protection rackets, prostitution, illegal gambling, loan sharking and drug trafficking, to break sworn oaths of loyalty and give evidence in a British court. He would help break a Triad criminal empire ranging from the Channel Islands to Glasgow.
'Who better to tell you what happened than the man trusted by the Triads to pull the trigger? He broke the oath which is at the very heart of Triad activity. So strong is their loyalty and fear of retribution that few, if any, have ever before been prepared to assist the authorities,' Martin Heslop, for the prosecution, said.
But the jury had trouble believing him. A self-admitted assassin, Cheung, 28, cut a strange figure in the witness box. He did not fit the stereotype of a 'hitman'. He was 5ft 4ins with thick lenses - which earnt him the nickname 'Specky' - yet he had been described as a 'sadistic assassin' and a 'thug steeped in crime'.
As details of his background emerged Cheung seemed more misfit than hitman. He was born in Leicester, where his parents owned a take-away, but they had disowned him, and letters written from prison described his resentment of his brothers and sisters. 'All I want in life is some love and respect,' he said, but his only real companion was a dog.
His grasp of English was better than Cantonese, and his letters also reflected his isolation. 'All my life I have been stuck in between the English and the Chinese. The English treated me as Chinese. The Chinese treated me as English. I haven't much chance of recognition in England, getting an English job and getting on.'
The court heard Cheung's acne, personal habits and scruffy appearance meant he was unsuccessful with women and regularly used prostitutes. He admitted smoking cannabis, taking LSD and drinking heavily.
The jury was told that as a bodyguard to Ngau Kor, the Triad boss, he enjoyed status and respect. He carried out ruthless attacks on several people to order. He stalked one of his victims before slashing his face with a double-bladed knife, which left the man scarred.
Cheung claimed he was frightened and unable to back out of the attacks, some of which were carried out around Britain, but he admitted his efforts won him 'recognition' from his bosses.
As evidence pointed to him as the man who shot Ying Kit Lam, a Hong Kong businessman in London's Chinatown, Cheung took the decision he must live with for the rest of his life. Knowing the penalty for informing on 'brother' Triads was death, he approached detectives for a deal.
Mr Heslop summed up Cheung's position: 'He will probably never have any hiding place. He is going to be on the outside for the rest of his life and will always be looking over his shoulder expecting a bullet. As an outcast he can never return to the Chinese community.'