Police constables like to say they have discretion. Judges clearly have. But do prosecutors? Not really, says their boss, Scotland's Lord Advocate.

"I prefer to speak of professional prosecutorial judgment," explained James Wolffe QC. "Prosecutors do not exercise discretion, rather they exercise professional skill in assessing the evidence in the light of the relevant law."

This may sound like a nicety. It is not. Ensuring justice is the same everywhere in Scotland is no minor matter. The line between prosecutorial discretion and prosecutorial arbitrariness, after all, is a fine one. And it is one which is often crossed on the continent.

Mr Wolffe, giving the annual APEX Scotland lecture in the Signet Library of the Faculty of Advocates, an another ancient body he once led, made it clear that he expects his prosecutors to make consistent decisions, wherever they are.

"Reasonable professionals may sometimes disagree," he stressed. "But I would not regard it to be desirable in our national prosecution service for different prosecutors to apply materially different approaches to similar offending, without good reason.

"Prosecutors exercise their framework within a framework which, today, seeks to secure reasonable consistency of approach across the system."

Mr Wolffe does not just mean that there are national guidelines. There are also national units, specialist teams of prosecutors in complex areas like sex crime who make sure prosecutorial decisions, including on how they lead police investigations, are broadly aligned. (Mr Wolffe he said he was "struck" to read French magistrates suggest very different attitudes to sex offenders).

The Crown Office, however, is not making decisions on whether to prosecute - or to prosecute on what charge - in a bubble detached from general public policy. It is not just the law it has to worry about.

Take decisions on whether to send an offender for some kind of diversion scheme - say drug treatment - rather than to court. A fiscal can only do that in an area where there is a properly resourced service. He or she cannot simply decide that somebody should be diverted to a project which does not exist or which is short-funded.

Thus the Lord Advocate, in an understated way, reminded his audience that the ability of prosecutors to divert an offender away from prosecution is "constrained by the diversity of provision" of such schemes.

Mr Wolffe welcomed the new national body, Community Justice Scotland, to oversee the whole system of non-custodial disposals - and try and end the revolving door of repeat offenders going in and out of jail.

The Lord Advocate - well aware of the "soft touch Scotland" - has made its own study of other systems.

Prosecutors may not have discretion. But they do have choice, choices that put them, Mr Wolffe says, in the "European mainstream."

Scotland has had a single police force for four years. It has had a single head of prosecution for four centuries. Can it be sure there is no post-code lottery for justice?