Post Office inquiry must examine rule on IT evidence if miscarriages of justice are to be avoided

Computer evidence rules

It is today seen as one of the biggest miscarriages of justice in British history, one that was made possible because of a change to the rules around computer evidence brought in at the same time the Horizon system was rolled out to thousands of Post Office branches.

There is a significant body of opinion, in IT and legal circles, that agrees that change to the guidance on the use of computer evidence in court must be made to avoid miscarriages of justice, like those experienced by subpostmasters. A better understanding of the role of rules on computer evidence could stir a public becoming increasingly aware of what happened.

In 1999, a presumption was introduced into law on how courts should consider electronic evidence. The rule followed a Law Commission recommendation for courts to presume that a computer system has operated correctly unless there is explicit evidence to the contrary. This legal presumption replaced a section of the PACE Act 1984, which stated that computer evidence should be subject to proof that it was in fact operating properly.

The new rule made it easier for the Post Office, through its private prosecution powers, to convict subpostmasters for financial crimes when there were accounting shortfalls based on data from the Horizon system.

As Computer Weekly revealed in 2021, following a response to a freedom of information (FoI) request made by computer scientist Steven Murdoch at University College London, the Post Office wrote to the Law Commission in 1995 when the law change was being proposed, and said the rule at the time, which said computer evidence should be subject to proof that it was in fact operating properly at the time, was “somewhat onerous” when prosecuting people charged with crimes, such as the subpostmasters that run and own its branches.

Response to Law Commission request

In response to a Law Commission request for feedback on proposals to change the rules, the Post Office wrote: “I consider that computer evidence is, in principal, no different from any other sort of evidence, and it should, in general terms, be admissible, so that any argument in court would be related to its weight rather than its admissibility. I therefore consider that there should be a presumption that the machine is in working order, etc, and if the defence wish to argue otherwise, then clearly, they should be able to do so. At present, I therefore consider the evidential requirements to be far too strict and can hamper prosecutions.”

It continued: “In the event of a subpostmaster being prosecuted for theft or false accounting, the Post Office may need to rely upon the computerised accounting records. The subpostmaster is frequently the only person who can give the evidence required by Section 69 of [PACE]. In the absence of admissions or other direct evidence the Post Office may not be able to prove the case solely on the ground of being unable to satisfy the technical requirements of Section 69 of [PACE].”

The Post Office was making clear that prosecutions would be difficult unless the computer evidence is presumed to be accurate. What is common knowledge today is that the Horizon software, which Post Office branches used, produced figures that were not accurate at all times. This data was used to successfully prosecute hundreds of people.

During a hearing at the public inquiry last week, this letter from the Post Office was highlighted by barristers, signalling that this will play a part in the current stage of the inquiry.

There have been loud calls for something to be done about this law, seen by one lawyer as “wrong-headed”. Lawyers, tech experts and politicians have called on government to replace it.

Last year, Kevan Jones, Labour MP and long-time campaigner for justice for the subpostmasters affected by the Horizon scandal, asked the secretary of state for justice whether the government planned to assess the legal presumption of reliability of computer evidence.

In response, Conservative MP and then parliamentary under-secretary of state for justice, James Cartlidge, said: “We have no plans to review the presumption, as it has wide application and is rebuttable if there is evidence to the contrary.” He said the current public inquiry into the Post Office Horizon scandal would advise the government on the matter.

At the time, Stephen Mason, a non-practicing barrister and editor of the practitioner text for judges and lawyers, Electronic Evidence, who has been trying to persuade the legal profession for over a decade to take the subject seriously, said the government could be waiting to reconsider the matter “should Horizon Inquiry chair Wyn Williams make any recommendations on this specific issue when he submits his report”.

Mason told Computer Weekly it is essential that the guidance on computer evidence changes if the inquiry is to reduce the risk of further miscarriages of justice: “Courts should be told to consider computer evidence in the context of the known errors in the software that produced it and the strength of evidence that the software was developed to high professional standards - evidence should be provided - and maintained and regularly audited to high professional standards.”

He added: “If the software was not developed to relevant professional standards, the known error logs will be incomplete, and the software and the evidence from it presumed unreliable and of no probative value.”

Ministry of Justice

When the government said last year it had no plans to review the presumption, Mason said that if the government does not intend to do anything, then the least the Ministry of Justice can do is implement the recommendations made by barrister Paul Marshall and others.

Marshall, who represented subpostmasters who successfully overturned wrongful convictions by the Post Office, was invited in August 2020 by MP Alex Chalk at the Ministry of Justice to submit a paper to the department on suggestions for improving the existing approach to the proof in court proceedings of computer-derived evidence. The paper was submitted to the MoJ and published. 

It recommended that when electronic evidence is used in legal proceedings, the party relying on the electronic evidence should automatically provide sufficient details of their systems to demonstrate that they are professionally managed.

Marshall told Computer Weekly that “it is difficult to imagine a more wrong-headed set of propositions” than the change to the rule on computer evidence that was introduced in 1999. He said the judgements of High Court judge Peter Fraser in the 2019 group litigation action, where subpostmasters proved that computer errors were to blame for unexplained losses, exposed “the fundamental flaws in the Law Commission’s analysis and in its conclusions. It also exposes, tragically, how strikingly unfairly the presumption that a computer is working properly may operate in legal proceedings.”

He said that since the change to the rules, it has been assumed as a matter of law “that the computer worked properly unless it can be shown that that assumption cannot safely apply”, and that this puts an evidential burden on someone, responding to computer evidence, that in many cases they are simply incapable of doing, “for the simple reason they do not – and cannot know”.

“The Horizon scandal in large part is the consequence of the Law Commission’s misconceived recommendations to parliament that PACE 1984 should be repealed,” said Marshall.

“The timeline of Post Office prosecutions matches exactly the period from which it was repealed. The Post Office had supported repeal – presumably to make its prosecutions for alleged ‘shortfalls’ easier and reversing the burden of proof was of immense practical value to the Post Office.

“The idea that problems with computers are usually apparent, as suggested by the Law Commission, is simply fatuous nonsense. The Boeing 737 Max disasters in 2018 and 2019 were due to software errors, which had lay dormant in the flight control software, probably for years. All computer programs contain bugs. Even safety critical software, such as in aircraft or medical equipment, has multiple bugs in it. These are facts that are insufficiently understood, as the Horizon prosecutions and false convictions all-too-painfully reveal.”

Insufficiently forensic

Richard Moorhead, professor of law and professional ethics at the University of Exeter, said: “Post Office cases show both it and the courts take an insufficiently forensic attitude to computer evidence and that while changing the law might well help, this is also about the courts being thoughtful about expert evidence and disclosure generally.”

The odds were stacked against the subpostmasters. Proving that the Horizon system was in error at the time of an unexplained loss would be almost impossible for most subpostmasters.

Speaking to Computer Weekly in 2021, after the government said it had no plans to review the presumption, Stephen Castell, computer software and systems expert witness and chairman of Castell Consulting, said the presumption “is and always was just plain wrong”. 

He challenged Cartlidge’s claim on behalf of the government that the presumption has wide application and is rebuttable: “While the presumption may in principle be rebuttable, that rarely happens effectively in practice, since the defendant has meagre resources compared with the state, the prosecution.”

When subpostmasters accused of theft or false accounting challenged the Post Office, it told them they were the only one having problems. If they persisted in challenging, the Post Office failed to give suspects access to evidence or outspent them in court, to win cases. When subpostmasters did receive expert help, it was often a different case, with the Post Office backing off and not prosecuting, through fear of the truth coming out.

Andy Clark, visiting professor in information security at Royal Holloway University of London and director at information security and expert witness company Primary Key Associates, told Computer Weekly in 2015 that 10 years earlier, he was called as a witness for the defence in a case brought by the Post Office against a subpostmaster. After seeing the Post Office Horizon accounting system in action, he said it was quickly apparent there were questions to ask about its integrity. After asking the Post Office these questions, the Post Office dropped the case, he said.

The Post Office knew that if evidence of Horizon errors got into the hands of subpostmasters it was prosecuting they would suddenly find it much harder to prosecute. A recent Horizon inquiry hearing revealed that the Post Office had a policy of not sharing evidence of computer errors with subpostmasters.

Internal document

A Post Office internal document for its staff investigating subpostmasters suspected of stealing warned that the inclusion of information on “significant failures” in reports that the suspected subpostmasters were sent, could reduce the chances of a successful prosecution of a suspect and/or cause “significant damage” to the Post Office’s business. It said this information about “significant” failures, including in regard to product integrity, must be “confined solely” to a report which was sent to its own lawyers.

From the roll-out of the Horizon system in 1999/2000 right up until the High Court GLO in 2018/19, the Post Office claimed there were no Horizon errors that could cause unexplained shortfalls. If subpostmasters had problems and suspected the computer was to blame, they were told they were the only one having problems.

During the public inquiry, former Post Office senior staff have claimed they were not aware of any errors with Horizon. One witness recently admitted being guilty of “absorbing the group think within the Post Office that [Horizon] was a good robust system”.

James Christie, an IT expert with extensive experience of software development, IT audit, testing and security management in the IT outsourcing industry, said it appeared the government was buying time and passing the buck to the Post Office Inquiry, when it announced it would not review the rule on computer evidence. 

He said the case for reforming the presumption is overwhelming, and “the government should have the resolve to initiate reform without waiting to see what Wyn Williams says.”

“If this presumption is not reformed, the Post Office scandal will not be the last,” said Christie. “It is long past time to act, and it is very disappointing to see the government playing for time.”

Murdoch at University College London said he would like to see the inquiry examine the effect of the presumption that computer evidence is reliable, in contributing to the miscarriages of justice and other problems related to Horizon: “While inadequate disclosure has been and continues to be a problem, the presumption has allowed the Post Office to prosecute subpostmasters without being required to support this with evidence as to the reliability of Horizon.”

The coming weeks are an opportunity for the controversial law to come into the gaze of the public.