The Post Office Horizon scandal is both simple and complex, straightforward but also difficult to grasp in all its interconnected detail.

The essence of the scandal is, of course, how groups of identifiable individuals within the Post Office - executives, lawyers and investigators - inflicted criminal legal liability and other horrors on unfortunate sub-postmasters and sub-postmistresses between 2000 and 2014, even though those executives, lawyers and investigators knew (or should have known) that the computer system on which they relied was flawed.

And when it became increasingly plain that there had been mass miscarriages of justice, groups of identifiable individuals within the Post Office and their external legal advisers sought to evade accountability and to cover-up what had happened, even employing extraordinarily aggressive litigation tactics.

But a saddening thing is that if it were not those particular identifiable individuals who were culpable (and they certainly should be held to account) then it would have been other individuals doing the same things. And this is because of legal and corporate contexts that facilitated this wrongdoing.

This is not to say that the guilty people can themselves blame the system - the culpable individuals could and should have done different things, at each and every step. Indeed, exceptional individuals could have stopped the nonsense and the cruelty.

But these were not exceptional individuals - they were individuals doing what they (wrongly) believed to be their job or performing what they (wrongly) believed to be their function or protecting what they (wrongly) saw to be legitimate interests.

As such it is not enough to identify and vilify villains, it also important to identify system failures that enabled such people to ruin the lives of so many innocent people.

Like the proverbial poor, the likes of Paula Vennells are always with us and will always somehow obtain senior positions. The question is what circumstances enabled this person and others to cause the consequences that they did.

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This post set out the faults of the legal system.

Indeed, like Marley being dead in A Christmas Carol, an understanding that the legal system being at fault is the basis of the story of the Post Office scandal.

Without the legal system being constituted as it was, then the Post Office could never have done what it did.

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The first legal fault is a rule of evidence: computer records must be presumed to be accurate unless shown to be otherwise.

Note that this is a presumption, not an absolute rule.

(And technically speaking the rule goes to admissibility of records as evidence, rather than the weight to be placed on them, but the practical effect is that once admitted the evidence is accepted for what it says.)

If computer records can be shown to be inaccurate, the presumption is rebutted.

The origin of this rule is from before the computer age: the days of mechanical instruments such as watches and then, more recently, speed cameras and breathalysers. The courts would presume such instruments were accurate and would allow prosecutions to rely on the evidence of such devices.

Such a rule of evidence makes sort-of-sense with such elementary contraptions, but it was (and is) woefully unrealistic with computer-generated data.

Indeed, software systems of any complexity are unlikely to be bug-free: the best one can hope for is that the bugs are not disruptive in any non-trivial way.

From 1984 to 1999 this rule of evidence had the presumption pointing the other way: it was for the prosecution to show the soundness of the computer evidence, and not the defendant. But this was repealed following a clumsy and ill-considered Law Commission proposal. The pre-1984 legal regime was reinstated.

If it were for the prosecution to show that the evidence was sound - with, say, certification on pain of perjury and full disclosure of error logs, and so on - then it is unlikely that many of the Post Office prosecutions could have taken place. It is certainly the presumption on which the Post Office relied upon in many prosecutions and in respect of which the Post Office resisted adverse disclosure.

In this way a mere rule of evidence created the conditions for horrendous miscarriages of justice.

There is no good reason for this presumption to remain. Prosecutions should only be able to rely on computer-generated records if (at least) on pain of perjury a director or similar figure certifies that there has been full disclosure of all relevant material to the defendant for the evidence to be properly tested.

No longer should any defendant be in the position of “computer says guilty”.

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Now we come to the next legal problem: disclosure.

Disclosure is a lawyer’s word for the process of one party providing another party with access to material evidence.

In general terms, it is the step in a legal case - either criminal or civil - when a case is won or lost. It is easy to assert a case based only evidence in one’s possession or control; it is only when the evidence is seen in the round that the merits of the parties’ respective positions can be keenly seen.

In civil cases, parties disclose their evidence to each other; but in criminal cases it is usually the prosecution that has the disclosure obligation. And the prosecution - rightly - has the obligation to disclose all evidence - even (especially!) the evidence that assists the defendant.

Prosecution disclosure of all material evidence to a defendant is thereby fundamental to the notion of a fair trial.

This is particularly so when a defendant has a presumption against them, such as those facing computer evidence.

For the only real chance a defendant has to rebut the accuracy of computer evidence - especially that evidence relied upon by a large organisation - is to have access to internal information held by that organisation. Unless internal information is disclosed the defendant is a lamb to the slaughter.

In the Horizon cases, the Post Office refused and resisted disclosure of the internal error logs that would have shown the system was unreliable.

It is not yet clear whether this was a deliberate and dishonest failing, or whether those with disclosure obligations simply did not know of information held elsewhere in the Post Office. Perhaps it was a mixture of both.

What is clear is that in at least some cases, the Post Office adopted tactics that meant that the Horizon system would not be called into doubt for other cases.

Whatever the true reason for this non-disclosure, its effect was stark. Defendants who would have had a defence had no defence; charges were brought which otherwise would not have been brought; lives were ruined needlessly.

The public efficacy of any criminal prosecution is only as good as the performance of key obligations by the prosecution, else (to use a techie term) it is garbage in, garbage out (or GIGO).

By not disclosing evidence as to the unreliability of the Horizon system, while benefitting from the presumption in favour of the soundness of the system, this meant that the Post Office had it both ways. Attempts by defendants to obtain disclosure were opposed by the Post Office and rejected by the courts.

The Post Office did what it did, because it could get away with it.

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And here we come to another glaring hole in the legal system: the use by the Post Office of private prosecutions.

The Post Office actually does not have any special standing to bring prosecutions; it has the same right to bring a private prosecution as any other person.

But the Post Office was prosecution-happy, with investigators boasting of holding interviews under the Police and Criminal Evidence Act.

Instead of prosecutions being a rare and extraordinary events, there were dozens, sometimes hundreds, a year after 2000, until they came to an abrupt halt in 2014.

The sub-postmasters and sub-postmistresses running post offices in their shops suddenly became a major criminal demographic.

When this happened perhaps senior managers at the Post Office would have thought that this grand explosion in larceny indicated that the evidence base may be faulty. But no.

Prosecute, prosecute, prosecute!

And because these were private prosecutions, there was no direct involvement by the Crown Prosecution Service and thereby no independent appraisal of the prosecution decision. Did the evidence available make out the offence? Was there a realistic prospect of a conviction on the evidence and the applicable law? Was a prosecution in the public interest? On these key questions, the Post Office prosecutors made their own, unchecked decisions, secure in the knowledge that the court would presume that computer records would be accepted as good evidence and that there would not be any disclosure otherwise.

In some cases - disgustingly - the Post Office ‘over’ charged with an offence of theft, even though there was insufficient evidence to show that the defendants were thieves, so that the (frankly) terrified defendants would plead guilty to the lesser offence of false accounting, which would normally avoid both a custodial sentence and the added stigma of a conviction for theft.

The Post Office simply got carried away with its role as an investigator and (private) prosecutor, and the convictions were easy because of the rule of evidence and the lack of disclosure.

Perhaps someone should have seen the convictions piling up and wondered what really was going on. But nobody cared. The computer said guilty, and the defendants had to be prosecuted, because they could be.

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Because of the magnificent stand made by Alan Bates and his fellow victims of miscarriages of justice (and because of the outstanding blogging of Nick Wallis), there was a possibility that the legal tide could turn - that the legal system that had treated the defendants so badly could maybe give them belated justice.

But the legal system did not make it easy for the victims; the legal system made it hard.

Bates and others launched civil claims against the Post Office - civil claims which the Post Office did everything to oppose.

These civil claims were likely to have got nowhere, had it not been for two strokes of legal luck - neither of which were inevitable.

The first stroke of luck was that the claimants were able to get the court to agree to a ‘group litigation order’ - in effect, a class action. These orders are rare and far harder to obtain than they should be.

And these orders are not ideal. To bring such a class action - even in a straightforward case - means a great deal of lawyery work and often needs funders who will put the money up for this work in return for the possibility of a return if the case is successful.

But when faced with the extreme legal tactics of the Post Office, however, this meant extensive work which would need to be funded.

A group litigation order made it possible for Bates and others to meaningfully take on the Post Office - but it also meant that any returns would be subjected to the need to pay funders and lawyers.

It was not an ideal litigation device - but it was the only one that would mean the claimants could take on the might of the Post Office.

Yet even a group litigation order did not mean success - and, given the Post Office’s aggressive litigation manner, the civil claims may have ended in defeat.

And then there was the second, game-changing stroke of luck.

The case was allocated to a High Court judge called Sir Peter Fraser.

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This is not a sweary blog, and so forgive the following characterisation.

Mr Justice Fraser was not going to take any of the Post Office’s shit.

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In a sequence of judgments, Fraser skilfully and painstakingly dismantled the Post Office’s legal and technical case.

(It is difficult to imagine many, if any, other High Court judges with the technical understanding and confidence to do this - and it is scarily easy to imagine many High Court judges instead nodding-along with Post Office counsel.)

In two judgments in particular - “common issues” and “Horizon issues”, both of which are forensic masterpieces - Fraser showed that the Horizon system was flawed and that the Post Office knew that the Horizon system was flawed.

During these hearings the Post Office took fright.

They instructed perhaps the most expensive barrister in the land to ambush the litigation mid-hearing with a sudden application to have Fraser recused for “bias”. They also sought (and presumably paid for) advice from a former President of the Supreme Court on how the cases before Fraser could be derailed.

There has never been a civil litigation ploy quite like it: it was as odd as it was desperate. It was, at best, a nuclear option. At worst, it was improper.

And it failed.

The Court of Appeal put the Post Office and its eminent legal counsel back in their post box.

Fraser could carry on.

And he did.

His judgment on the “Horizon issues” is lengthy, but it should be read by anyone interested in the law and/or the technical side of the case.

The Post Office now had nowhere to hide.

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Following on from the Fraser judgments, the Post Office position effectively collapsed.

The Court of Appeal began to overturn the convictions of those wrongly convicted - though because of an under-resourced and overly legalistic Criminal Convictions Review Commission, cases were only referred if the commission was certain they would succeed, rather than letting the Court of Appeal do its job.

The Post Office begrudgingly agreed to a settlement which, because of the nature of a group litigation order, meant considerable amounts went to the lawyers and funders of the claims, rather than to the claimants.

You would have thought that was it, but no.

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The innocent victims continued to campaign, slowly getting more and more press interest.

(And the then-head of the Post Office, who oversaw many of the wrongs, was at this point awarded a CBE and appointed by the current government to the Cabinet Office.)

The government set up an inquiry under a retired senior judge, so as to get to the bottom of the affair. Few noticed or cared.

And pretty soon the Post Office was back to its old non-disclosure and other evasive tricks. Just as the Post Office failed in its disclosure obligations in the original criminal cases, it began routinely to fail in its disclosure obligations to the new inquiry.

GIGO.

And this is where the position was until a few weeks ago: claimants under-compensated; the wrongly convicted waiting endlessly for acquittals; the Post Office playing its usual non-disclosure games.

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Then, this happened:

It is remarkable what difference a television programme can make.

Suddenly politicians urgently told each other (and voters) that something must be done, despite there being no new material facts since the Fraser judgments.

Perhaps the wrongs can be righted - at least for those victims who are still with us.

Perhaps the culpable individuals at the Post Office can be vilified.

But in all this noise, there is another issue.

What can be done about a legal system that makes it very easy for entities such as the Post Office to destroy lives but also makes it very difficult for those injustices to be corrected?

There were many individual failures - but there were systems failures too.

And until and unless those systems failures are addressed then further injustices will occur again, only with different culpable individuals.

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This is a re-written and extended version of an article at Prospect magazine.