Since the death of Myra Hindley, the British mass-circulation press has been in mourning. It has found it hard to come to terms with the absence of a living female figure of totemic evil to play the part in the popular imagination once occupied by witches.

Now, however, someone has emerged who might fit the bill ? even though she has been acquitted of murder and manslaughter. This is the mother of Baby Peter, the angelic-looking blond toddler who suffered what must have been a slow and agonising death as a result of brutality in the home.

Two children a week, on average, die in this country as a result of abuse in the home: sadly, the only remarkable thing about the Baby Peter case was the political and public obsession with it ? a consequence of the fact that the death was the second in recent years of a child known by Haringey council to be at risk. Against this background it was always likely that the sentences handed down to those involved in Baby Peter?s death would in turn be the subject of a frenzy of attention ? and so it has proved.

The Daily Mail and The Sun have mounted instant campaigns of outrage against what they see as the leniency of the sentences handed down by Judge Stephen Kramer nine days ago. Jack Straw, the justice secretary, initially held the line, stating: ?The judge has been clear about the minimum terms they must serve, and such decisions must be for the judiciary.?

Characteristically, the government has now retreated in the face of a further media barrage, and the office of Baroness Scotland, the attorney-general, announced last week that she had ?called for the papers in this case since [she ]has the power to refer certain sentences to the Court of Appeal, if after looking at all the facts she thinks the sentence was unduly lenient?.

So here are the facts. It is a recent law, passed in 2004, that obtained a custodial conviction for Baby Peter?s mother. Before it was passed, in fatal child abuse cases where each of two partners denied guilt and blamed the other, and neither could be proved to have caused the fatal injury, there was little the courts could do to hold either (or indeed both) directly responsible for the killing.

Baby Peter?s mother and her boyfriend were convicted under the 2004 act, which introduced a new offence of ?causing or allowing death?. Kramer sentenced Baby Peter?s mother to a ?minimum? term of five years in prison, which meant little more than three when the almost two years she had already spent in custody were taken into account.

This is what has caused so much outrage. However, the public does not seem to have been made aware of the other aspects of Kramer?s sentencing of Peter?s mother. He, in fact, declared a sentence of 10 years, but under the government?s own rules ? which automatically release prisoners by executive order after they have served only half their sentence ? that arbitrarily comes down to a custodial ?minimum? of five.

Kramer further indicated that he would have passed a higher sentence had not Baby Peter?s mother pleaded guilty to the charge of ?allowing? the death of her son and had she not on two occasions sought medical help for Baby Peter unprompted.

Most significant of all, the judge also passed on her an ?indeterminate [sentence] of imprisonment for public protection?. What this means is that she will never be released unless, as the judge put it, ?the parole board determine . . . you are deemed no longer to be a risk to the public and in particular children?.

The indeterminate sentence for public protection is also a recent form of punishment ? it was proposed by David Blunkett as home secretary and became law in 2005. At the time it was suggested that it would apply to only a small number of offenders, but since then judges have handed down almost 11,000 ?indeterminate? sentences, and so far fewer than 50 such prisoners have been released. Some indeterminate sentences have been imposed on people whose ?minimum? term is less than 12 months: such prisoners are now known as ?short-term lifers?.

It is not hard to guess why judges have taken up these sentences with such enthusiasm. Not only are they under perpetual pressure from the Home Office to keep fixed sentences as low as possible because of overcrowded prisons, but they also know that any fixed sentence they pass (itself limited by guidelines) is immediately halved by executive order.

In other words, they are statutorily prevented from ensuring that the punishment fits the crime. No wonder many of them seize on indeterminate sentences as a way of passing the buck to the parole board. Then, if the released prisoner reoffends violently after he has been judged by the parole board to be ?no longer a risk to the public?, it is not the judge who will be held responsible.

It is in any case simply a bureaucratic fiction that any parole board knows the likelihood that a released prisoner will reoffend. My wife?s cousin, John Monckton, was murdered by Damien Hanson in 2004, only weeks after Hanson had been released halfway through his sentence for attempted murder, following a parole board recommendation. The board had been impressed by the way that Hanson had attended ?anger management classes? in prison: the fact that he would have known such attendance was the sort of visible act that impressed parole boards shows how vulnerable the system is to manipulation.

By contrast, the wrongfully convicted man who refuses to acknowledge his ?guilt? can be kept in prison indefinitely. The continued pleas of innocence by Sean Hodgson, released in March after 27 years for a murder he did not commit, had kept him inside long beyond the time served by many dangerous killers ? it was only the advent of DNA evidence that showed he had been telling the truth.

One prison psychologist told me, after many years of bitter experience: ?We know that a small proportion of prisoners we release on probation will go on to commit rape or even murder; the trouble is, we have no idea which ones they will be.?

He is contemptuous of the fashion for short minimum terms accompanied by an indeterminate sentence, arguing that such custodial packages ?don?t take sufficiently seriously what the offender has done, and take too seriously what he might ? and therefore might not ? do?.

It is very different from the American system, in which indefinite sentences are becoming rare and long fixed sentences without possibility of early release are common. This is at least honest and open: the hallmarks of good justice. Both the public and the criminal know exactly how things stand, which is not the case with the paradoxes and evasions of the English system.

The advocates of our system maintain that it is more humane, since it allows for the prisoner who displays penitence to be released much earlier. Sixty years ago CS Lewis demolished this conceit in his essay The Humanitarian Theory of Punishment. He pointed out that a sentence based solely and inflexibly on the wickedness perpetrated ? the concept of just desert, which was increasingly being denounced as ?mere retribution? ? was the only way of linking punishment and justice.

By contrast, said Lewis, if sentences served were based on a subjective assessment of the rehabilitative process, ?grumpy unrepentant prisoners? could be consigned to perpetual incarceration while those cunning enough to ?cheat with success? would be freed.

The dystopia foreseen by CS Lewis is now the English system of justice.