It is illegal, but would most of the public consider the theft worth the expense of a trial?

What about one of a horde of boys stripping trees of their fruit, leaving the farmer facing ruin and the community without apples?

This is the example Mark Lynn, the head of Warwickshire's Crown Prosecution Service, uses to demonstrate a key principals guiding who should be prosecuted and when.

He thinks most members of the public would agree the CPS should pause before hauling the starving youngster before the courts. It should act in the public interest. He notes that in some places there is no discretion not to prosecute.

"Hordes of boys stripping trees of apples is different, but in some countries it is not treated as such," he said.

Yet Mr Lynn says only that the service "usually" makes the call the public would. Administering the law's in-built assessment of the public interest is clearly sometimes a tricky proposition, where the public interest, or good, is not always the same as what it wants.

He added: "If a person is facing seven years for robbery, the fact his car had no MOT is not of the greatest concern to most.

"But take the serial murderer already facing death in prison. Should we still prosecute all of their offences that come to light? What about the victims' families? Consideration must be given to each case; the argument must be whether a prosecution is justified in light of the likely results - in all respects."

The service is largely policy driven - it responds to a decision made by the Director of Public Prosecutions when it pursues anyone caught with a knife, not directly to "the more popular press". But there is space for a little local variation in response to more proletariat demand.

Mr Lynn added: "Earlier in my career, in Crawley, we had an estate where people riding bicycles on the footpath was a huge problem. Elderly people were being mowed down. In that situation, though you might not prosecute usually, there was an element of doing so 'pour encourages les autres'."

Of course, public interest is only half of the equation in determining who faces the courts.

The prosecutor must also be satisfied there is enough evidence to provide a "realistic prospect of conviction".

One area where that may shortly be threatened is the use of anonymous witnesses, which a recent Law Lords ruling appeared to undermine.

Mr Lynn says this has not presented a problem to ongoing prosecutions in Warwickshire, "but it could do". He added: "In particular it could affect cases of gang violence, where finding people prepared to speak out can be difficult. And there are two sides here - the civilian witness is affected, but so too the undercover police officer. What is not negotiable is the fair trial. My job is to prosecute, not persecute.