A crowd of 200 had gathered to watch the prisoner arrive. Mothers and fathers with pushchairs and small children in tow, screamed 'Scum', and 'Die' and 'Kill the bastard'. A man built like a Charolais and dressed in sweatshirt and trainers hurled himself at the silver police people-carrier bearing Roy Whiting, a 42-year-old mechanic, to Chichester Magistrates Court, where he was remanded in custody last Wednesday, charged with the kidnap and murder of Sarah Payne.  The fury of the mob had been stoked at first by horror at the killing of an eight-year-old and by the News Of The World 's subsequent name-and-shame campaign. More recently the police, if not whipping up hysteria, have, seemingly, done little to tamp it down. A week ago, detectives helpfully tipped off the press that they planned to charge Whiting, who had been arrested and questioned twice previously, within three days. It was believed, though not confirmed, that their breakthrough hinged on an appeal by the BBC's Crimewatch programme.   As a 'senior police source' told the Daily Mail: 'The incident room has not stopped working since the day little Sarah disappeared. The man is due to be arrested... and the mood among many officers is buoyant.' If true, this revelation of police hubris is a disgraceful affront to justice.  Roy Whiting is innocent in the eyes of the law. He has entered no plea and faced no jury. Once a quaint old protocol decreed that, at the whiff of an arrest, a veil of secrecy descended. Now, it seems, practically anything goes in high-profile murder cases, where tragedy elides with show business and the public clamours for results.  Can Whiting get a fair trial, given the weight and type of publicity surrounding Sarah's murder? Among other examples of goldfish-bowl defendants, the next in line is Barry George, a prime focus of the media spotlight, who goes on trial for Jill Dando's murder in a fortnight.   One can see why three Appeal Court judges resisted the argument that Michael Stone should not have his case tested again in court because, as his barrister claimed, 'the wealth of prejudicial material... would make such a retrial unfair'. Perhaps, given the tonnage of pre-publicity surrounding criminal cases, the best to be hoped for is an amnesiac jury.  Later this week, the judges will give their reasons for ordering a retrial following their decision to quash Stone's conviction for the murders of Lin Russell and her daughter, Megan, and the attempted murder of Josie Russell, after a key witness had been shown to be unreliable.   The Russells will have to revisit their tragedy. Kent police may, in due course, have some tough questions to answer as to why justice miscarried. For now, the best that can be said of the whole debacle is that the appellate process has been shown to work. Whether Stone is found guilty or innocent, his case was treated with a swiftness denied to others.   Last week Stephen Downing finally walked free, after serving 27 years for the murder of Wendy Howell, a crime he almost certainly did not commit. Disgracefully, he was held in prison long after he would normally have been released, because parole boards refuse to free those who do not admit their guilt.  Last week too, the case of a man hanged for the murder of a cinema manager in 1950 went to appeal after the solicitor for George Kelly's family argued that 'the police and courts believed it was more important to secure a conviction to reassure the public than to properly investigate a crime'.   So what's changed? A quarter of a century after Kelly went to the gallows, the notorious West Midlands Serious Crimes Squad began its regime of torture, bogus confessions and creative note-taking, resulting in a litany of miscarriages of justice. The Birmingham Six were wrongly convicted, and Ann Whelan spent 17 years fighting for the release of her son, Michael Hickey, and watching his life unravel in prison before it finally emerged that he and the two other living members of the Bridgewater Four had been framed by the Squad. How long ago it seems since the wrongful conviction of Judith Ward, Stefan Kiszko, the Maguire Seven, the Guildford Four. How far we have come since then.   But not, perhaps, quite far enough. The Criminal Cases Review Commission is over-loaded with petitioners claiming to have been wrongly sentenced, and the Crown Prosecution Service remains unaccountable, prone to arbitrary decisions and strapped for cash. While science should make convictions watertight, the perception - or the reality - can be quite the opposite. The pattern of invisible blood spots that clinched the case of Sion Jenkins, the teacher found guilty of murdering his step-daughter, Billie-Jo, did not impress a number of powerful advocates who argue that he should never have been convicted. That unease, and the worries raised by the Michael Stone case, don't, of course, denote a return to the corrupt ways of the Seventies.   There are, however, new considerations in an age when locking people up is becoming so simple. Not long ago, Tony Blair announced plans to take DNA samples from everyone arrested in Britain. Fine, except that genes are already being interpreted not only as evi dence of a crime committed but as indicative of one that might be. If genetic profiling becomes part of police investigation, then the presumption of innocence dies. The Law Commission is expected soon, in the light of DNA advances and Sir William Macpherson's recommendations, to relax the 'double jeopardy' principle, under which an acquitted suspect cannot be retried for the same offence. Fine, unless the police and CPS produce slovenly prosecution cases, knowing they can always have another bash.   But the really iniquitous development is the promise by Government of post-election legislation indefinitely to detain people with untreatable personality disorders. Though the real scandal is the dearth of psychiatrists and of hospital beds for the mentally ill, Jack Straw has for years preferred to dream of incarcerating those who have committed no offence but who might. His success, framed in a recent White Paper, may delight those who choose not to recognise that schizophrenics commit 10 murders a year, or about 2 per cent of all killings. We are 20 times more likely to be killed by a sane person than an insane one, but such statistics carry little weight in a hot climate of fear in which we are encouraged to believe that a child-molesting monster lurks at every corner.   In fact, British children are the second safest in the world, according to a new survey by Unicef. But even if they were not, there would be no justification for such a gross affront to human rights. When the weight of the criminal justice system offers no guarantee that we are locking up the right suspects, incarcerating people while skipping the optional extras of a crime and a trial is hardly likely to produce good outcomes. And besides, for those failed by the courts, there is still the prospect of finding justice, even if, in Stephen Downing's case, the quest took most of his life. Jack Straw's and Alan Milburn's detainees, unconvicted but 'untreatable', may have no such luxury.