In deciding that the tariff imposed on the paedophile Craig Sweeney was not "unduly lenient", Attorney General Lord Goldsmith was not exercising his perception but interpreting statute law.

The concept of an appeal against a sentence considered to be too light was introduced by the 1988 Criminal Justice Act.

The sentence must "fall outside the range of sentences which the judge, applying his mind to all the relevant factors, could reasonably consider appropriate".

The definition of "reasonable factors" has been fleshed out over the years, most recently - and controversially - in the 2003 Criminal Justice Act.

Under Section 144 of the Act, a sentencing judge is required to take into account the stage at which a defendant pleads guilty and the circumstances in which an intention to plead guilty is indicated.

The act, in section 172, places a duty on the judge to pay regard to any relevant sentencing guidelines.

The government's purpose was to encourage early guilty pleas in order to spare victims and witnesses the trauma of giving evidence.

It was also hoped this would save the criminal justice system vast sums of money spent needlessly every year on setting aside court time for contested trials which don't take place because defendants eventually plead guilty - sometimes on the very day of trial. Such occasions are known as cracked trials.

In that, it has been successful.

No alternative

Many people can't understand why someone who seemingly has little choice but to admit guilt, because the evidence is so overwhelming, should get the maximum one-third discount.

Lord Goldsmith reflects this view.

In June 2006, he argued in the case of the convicted sex offenders, Alan Webster and Tanya French, that photographs of abuse perpetrated by the pair against a baby girl had left them no alternative but to plead guilty and that they should not get the maximum discount.

A rare five-judge hearing of the Court of Appeal, headed by the Lord Chief Justice, ruled against him. They said that the sentencing guidelines expressly stated that credit should not be withheld or reduced where the offender was caught red handed.

The Lord Chief Justice urged a review of the guidelines as a matter of urgency and it seems probable that this will result in more discretion for judges when deciding the tariff to be served.

When the 2003 Act was being drafted, the chief concern was about the length of time served by convicted killers.

Now, the focus has shifted towards other serious crimes, particularly child sex abuse.

But it has to be acknowledged that, however finely calibrated the guidelines, sentencing will always be more of an art than a science.