By the time Dean Boshell's blood-soaked body was found in allotments on the outskirts of Leigh-on-Sea in Essex, rigor mortis had set in. He had been shot once in the head before the killer fired twice more into his temple. The death of Boshell, a 24-year-old petty criminal and police informant, in February 2001 took detectives from Essex police into the heart of the violent and feuding criminal gangs that competed for control of the drugs scene around Southend. When a conviction for Boshell's murder was eventually secured, they made legal history with their use of unprecedented supergrass evidence. Prosecutors described the case as unique: the first time a murder charge had been dropped against one individual, who then went on to become the crown's star supergrass witness against another man who was ultimately convicted. Six years on, however, the activities of Essex detectives and prosecutors, and the lengths they went to in order to convict someone for the murder, are coming under scrutiny – along with a string of other convictions which have relied solely on the uncorroborated word of supergrasses. In the murky world of the supergrasses, most of whom are criminals who snitch on former friends to cut their own jail time, the case of Damon Alvin is unsurpassed. He started his criminal career in his early teens and by his mid-20s was running a mini drugs empire from his dormer bungalow in Benfleet, Essex. Over the years he had been convicted of several offences and been in and out of prison. He had convictions for violence, burglary and drugs offences, and police intelligence showed he was a skilled liar who was involved with firearms. It was during one of his frequent spells in jail that Alvin met Boshell and the pair became close; but the relationship was not on equal terms. According to court papers, Alvin treated Boshell as a gofer, while the younger man looked up to him as a brother. When Boshell was found dead on a cold morning in February 11 years ago, it was a sign to police that rivalries between groups of criminals engaged in supplying drugs in the region had boiled over. Detectives turned first to known associates of the dead man: Alvin, and another known drug dealer, Ricky Percival. Percival had built a drug-dealing business around individuals he met within the Essex bodybuilding scene, according to legal sources close to the case. But during a frustrating inquiry, which for years seemed to go nowhere, it was Alvin that the police focus returned to repeatedly. Arrested three times in three years for the murder, he was often pressed to talk and name others who might have been involved, but always refused. In 2003, after he had been arrested for supplying 1kg of cocaine, detectives recruited him as an informant in their efforts to disrupt local drug gangs, according to court papers. After his third arrest, detectives charged Alvin with the Boshell murder on the basis of mobile phone evidence which proved his alibi for the night was a lie. The mobile footprint revealed Alvin had driven to Southend to collect Boshell on the night he died – something he had never mentioned. Percival and two others were charged with conspiracy to pervert the course of justice; the police alleged they had made up an alibi for the night. What happened as Alvin faced his murder trial is now being scrutinised by the Criminal Cases Review Commission (CCRC). Two weeks into the trial at Chelmsford crown court, while lawyers were arguing about admissible evidence, Alvin indicated he wanted to change his story. He spoke out after a successful application by the prosecution to admit into evidence police contact sheets which would reveal to a jury that the dead man was a police informant, who had been passing information to detectives about Alvin for several serious offences – thus providing a murder motive. The trial was halted and in interviews with the police, Alvin accused Percival of being responsible for Boshell's death. Alvin admitted for the first time that he had been at the allotments on the night of the murder, but said he had stood by as Percival first threw bleach in the victim's face, before shooting him dead. Alvin also implicated Percival in a string of other crimes he himself was suspected of, and later admitted to, including the attempted murder of three people in a gangland feud. The police will not comment on the case but are likely to argue that Alvin – facing possible conviction for a crime he had not committed – finally agreed to inform on the individual responsible at the 11th hour. Percival, however, says Alvin did his deal with the police to escape the murder charge and in doing so fingered an "innocent" man. Peter Hughman, a solicitor who has acted for Percival, said: "There is something strange in relying on uncorroborated evidence from a person of simply appalling character." Eight days later, the Crown Prosecution Service sent Alvin a letter announcing the Boshell murder charge was being dropped. He became the crown's star supergrass witness against Percival, and entered into a deal with police and prosecutors which required him to confess all his criminality. Over five and a half months, Essex detectives spent 94 hours interviewing Alvin. In December 2006, after an 11-week trial, Percival was convicted on the basis of Alvin's evidence of the murder of Boshell and given life with a recommendation he serve a minimum of 28 years in prison. Today Alvin is living under a new identity, having been relocated following two and a half years in jail for a string of offences he admitted as part of the agreement he entered into with the police. Over the months he was held in a safe house during his supergrass interviews, police documents show officers spent nearly £35,000 on him – including £7,125 towards a new car and £468 on a laptop. He received money to top up his mobile phone, pay parking fees and buy an enclosure for his tortoises. Alvin also benefited to the tune of £190,000 from the sale of his house, allegedly facilitated by the police while he was in jail. Land Registry records show the property changed hands on 22 November 2006 for £250,000. It is understood there was a £60,000 mortgage on the house. A source close to the process said: "The police were always in the background of the sale." Under the Proceeds of Crime Act, the Essex force could have seized the money from the sale, but no seizure was made and the proceeds went to Alvin. Now 32, Percival is serving his life term at a B category prison, Swaleside in Kent, from where he has been protesting his innocence for six years. Percival – who learned to read and write in prison – told the Guardian: "I still cannot really believe what happened to me. When I first came into prison I was in some kind of intense shock: I couldn't sleep, I was having nightmares, I was turning it all over in my mind – how could this happen in the British justice system? "They had to put me on medication because I was suffering from such anxiety and shock. And today it still feels the same, the shock is as raw. "What makes it even harder is that anyone who has a good understanding of my case says: 'How did a jury convict you?' " Percival's claims of innocence were bolstered recently when he passed a lie detector test – the results of which are being considered by the CCRC. Much derided in the past, the accuracy of polygraph tests has improved so much that they are being evaluated by at least one police force, and the Association of Chief Police Officers could extend the trials across the country. Percival, who admits he played a major role in the Essex gangland drug world, lost his appeal last year. The court of appeal said the case was unique, in that it "relied for its essence on the evidence of a witness, Alvin, who had been charged but acquitted on the same murder as that on which Percival was tried … [The case] stood or fell on Alvin's evidence." But after considering the case, the judges ruled that there was a "richness" to Alvin's story which made it believable, and they refused the appeal. Those close to Percival say this "richness" comes from the months in which Alvin studied the papers from the case while on remand. Evidence in court documents reveals that prison officers said Alvin's cell resembled a police incident room, papered with case documents, timings, maps and testimonies. "Damon Alvin used his year on remand to develop this false story which guaranteed his freedom," said Percival's mother, Sandy. "The records of his statements to police show his story has changed several times. Our argument has always been: show us the corroborative evidence that supports Alvin. There isn't any. "My son has done wrong, I know that, but he is not a murderer. There is no proof that he did any of these things, it is all Alvin's word against his. If I thought my son was a murderer, I would never stick by him." Documents held by the CCRC highlight another key challenge to Percival's conviction. Under the terms of the witness protection scheme Alvin entered into, he had to confess all his criminality. Yet court papers show he did not reveal one key element to his criminality that could breach the protection agreement he entered into, and which raises questions about his credibility as the crown's star witness. Alvin failed to tell police he had previously lied in court – potentially perjuring himself – in order to gain a reduced sentence for possession of 1kg of cocaine. Facing a seven-year sentence, Alvin fabricated a story that his life was under threat from four unnamed gang members, court papers show. To colour his story, he arranged for his wife and mother-in-law to cut out letters from newspapers to create threatening notes, and told another family member to send him a wreath at home, to supplement his claims. He also used hospital records from an injury he received during a domestic row as evidence that he had been attacked. Alvin's story was enough to convince a judge at Basildon crown court, who gave him a reduced sentence of 30 months for the drug offence. Hughman said the CCRC should examine the whole issue of supergrass evidence, and called on the court of appeal to issue guidance on such witnesses. "There should be a continued public interest and questioning about using people who are so manifestly unreliable to secure convictions, even if the authorities are particularly anxious to convict certain people. It doesn't make for safe convictions." The CCRC said it was still examining the case and would not comment. The Guardian made repeated requests to the CPS but it also refused to comment on the case. Detective Chief Superintendent Liam Osborne, of Kent and Essex serious crime directorate, said he would not comment while the case was with the CCRC.