In 2015, the sub-postmistress at the Dalmellington branch moved £8,000 in cash from her core branch to its outreach branch, a satellite counter serving a remote community. She scanned the pouch once. Horizon, the Post Office’s computer system, recorded the transfer four times. Her accounts now showed £32,000 arriving and a £24,000 hole.
She was lucky, a High Court judge later wrote. She ran both ends of the transfer, so she could show where the money really was. Fujitsu, which built and ran Horizon, investigated and found the same fault had struck more than 100 times at 88 branches since 2010.
Most sub-postmasters had no such luck.
They run post offices under contract, often inside their own shops. From 2000 every branch had to use Horizon, and there was no way inside the system to dispute its figures. If the cash came up short of what Horizon said, the sub-postmaster had to make up the difference, from their own money or by borrowing. Some who couldn’t, or who covered the gap on paper, were charged with theft or false accounting. The Post Office itself prosecuted 700 people between 1999 and 2015.
From 2000 on, sub-postmasters had been reporting shortfalls they couldn’t explain. Head office had its answer. In August 2010 an internal report called Horizon robust, and of the prosecutions that had drawn criticism it said: “we remain satisfied that this money was missing due to theft in the branch.” It weighed an independent review, then warned that “any perception that POL doubts its own systems would mean that all criminal prosecutions would have to be stayed.”
Even Dalmellington didn’t change the line. On 1 July 2016, after a blog post about Horizon errors, chief executive Paula Vennells emailed two senior colleagues: “This needs looking into please.” One of them asked for an urgent review. Later that same day he wrote again: “Can you stand down on this please?” The emails are heavily redacted, and no Post Office witness explained the reversal. Later that month, the Post Office’s lawyers told the sub-postmasters preparing to sue that its investigations kept pointing to human error or dishonesty in the branches.
In 2017, 555 sub-postmasters led by Alan Bates took the Post Office to court. At trial, its barrister compared bugs to lightning: across a big network, a few people will be unlucky, and you can still reason about the odds. The claimants’ IT expert answered that at Dalmellington, as he recalled it, one branch had been hit five times. “It would be very unusual,” he said, “for one person to be hit by lightning five times.”
In December 2019, Mr Justice Fraser ruled. The experts’ table listed 29 suspected bugs, and by the end of the trial the Post Office argued that only eight of them were not bugs at all. The original Horizon, he found, was not robust. The Post Office’s refusal to consider any other explanation, he wrote, amounted to “the 21st century equivalent of maintaining that the earth is flat.” It had already settled with the claimants for £58 million, much of which went on legal fees.
In April 2021 the Court of Appeal quashed 39 convictions and found every one of those prosecutions an affront to justice. In May 2024 a new law quashed hundreds more convictions that met its conditions, among them that the Court of Appeal had not already considered them. The Court of Appeal’s judgment explains why the certainty lasted so long:
“As each prosecution proceeded to its successful conclusion the asserted reliability of Horizon was, on the face of it, reinforced.”