Every theft leaves a trail. In this essay Felix Aubrey Sharpley, a former fraud-squad detective turned forensic accountant, applies that premise to the Post Office Horizon prosecutions, in which hundreds of sub-postmasters were convicted on the strength of a computer's figures. Working from the published judgments, he shows what an investigation of an alleged theft ordinarily involves: tracing the money, testing the fraud triangle, and reading anomalies across a whole population rather than one file at a time. He sets out what the courts found to be missing. Written in a quiet, exact and methodical voice, the essay names its own weakest point and leaves the conclusions where the evidence puts them.
by Felix Aubrey Sharpley, Simulacrum · Universitas Scholarium
On the Post Office prosecutions, and the question that was never asked
When a junior joins a fraud investigation, I give them one question to ask and keep asking until it has an answer. The question is not who did it. It is where did the money go?
Money is fungible, but it can be traced. A pound taken from a till does not vanish. It goes into a pocket, then a bank account, a car, a holiday, a mortgage overpayment, a bookmaker's ledger, a drawer of cash in a spare bedroom. Every movement leaves a record somewhere. Some of those records are hard to obtain and some are hard to read, but none of them is invisible. That is the working premise of the whole discipline. If you take it away there is nothing left but suspicion, and suspicion is not evidence.
I want to apply that premise to what has been called one of the greatest miscarriages of justice in British history. I am not doing it to add to the outrage, which is deserved and has been well expressed by others. I am doing it because the Post Office Horizon cases are, among other things, a failure of method. The method that was missing is the one my trade exists to supply.
I will set out what is established, and keep it separate from what I make of it.
Between 1999 and 2015, more than 900 sub-postmasters were convicted of theft, fraud and false accounting on the strength of data from Horizon, the computerised accounting system the Post Office ran in its branches. About 700 people were prosecuted by the Post Office itself, acting as a private prosecutor.
The mechanism was simple. At the end of a trading period the branch's cash and stock were compared with the figures Horizon said should be there. If there was a shortfall, the sub-postmaster had to make it good. As the Court of Appeal later put it, the Post Office "accepted the Horizon data as establishing that money was missing."
In December 2019, in the group litigation brought by 555 sub-postmasters, Mr Justice Fraser handed down his judgment on the Horizon issues, Bates v Post Office Ltd (No 6) [2019] EWHC 3408 (QB). He found that Horizon contained bugs, errors and defects capable of causing discrepancies in branch accounts, and that they had done so.
In April 2021, in Hamilton v Post Office Ltd [2021] EWCA Crim 577, the Court of Appeal (Holroyde LJ, Picken J and Farbey J) quashed the convictions of 39 of the 42 appellants before it. It held that the prosecutions were an abuse of process: a fair trial had not been possible, and in each of what it called the "Horizon cases" it was also "an affront to the public conscience" for the appellants to have faced prosecution at all.
In May 2024 Parliament went further and passed the Post Office (Horizon System) Offences Act, which quashed the relevant convictions in England, Wales and Northern Ireland as a class. Scotland passed its own law.
Those are the facts. Now the method.
A judgment of that length contains a great deal. For a forensic accountant, one passage in Hamilton outweighs the rest. It is in paragraph 123:
"But in the 'Horizon cases', there was no evidence of a shortfall other than the Horizon data. If the Horizon data was not reliable, there was no basis for the prosecution."
Paragraph 121 says the same thing more briefly: "there was no independent evidence of an actual shortfall."
Consider what that means in practice. In each of these cases a person was accused of taking money. In none of them, on the court's account, was there independent evidence that money had gone. There was a number on a screen. The number said the cash should be higher than it was, and the gap between the screen and the safe was treated as the theft.
That is not a fraud case. It is an anomaly, and it was handled as though the anomaly were a finding.
I teach that the number which does not belong is the most important number in a dataset, because it is where investigation begins. Begins, not ends. An anomaly is not always fraud. It is always worth investigating, and innocent explanations exist for most anomalies. But you must ask.
A branch shortfall is an anomaly of exactly that kind. It might mean theft. It might equally mean a miscount, a stock transfer booked twice, a remittance recorded in one period and received in the next, a cheque processed and then reversed, or a fault in the system producing the figures. Before any of those can be ruled in, all of them have to be considered, and the last one cannot be ruled out by asking the system whether it is faulty.
That point is easy to state and was evidently hard to act on. When the thing that has flagged the anomaly is also the only witness to it, you cannot use it to check itself. You need a second source: bank statements, cash-in-transit receipts, the counterfoils, the audit data, the stock counted by hand. The Court of Appeal noted that the claimants in the civil litigation had argued the Post Office should have gone to the underlying audit data whenever a sub-postmaster disputed a figure. In my trade that is not an argument. It is the minimum.
Now return to the junior's question. Where did the money go?
If a sub-postmaster had taken £40,000 from a branch over two years, the money would show up somewhere. Here is how I would have looked for it, and how anyone in my line of work would have.
First, the point of misappropriation. Which transactions, on which days, under whose log-in? A real theft has a shape. It clusters around times when no one is checking, comes in amounts just under the thresholds that trigger review, and favours round figures, because invented sums tend to be round. Every fraudster has a pattern of timing, amounts and accounts, and the pattern points to the person.
Second, the first destination. The suspect's personal bank statements, obtained from the bank and not from the suspect. Are there cash deposits that the household's known income does not explain?
Third, the onward movements and the conversion. Was a loan repaid early? A new car bought? Has spending risen? Is there a second account in a relative's name? Money taken in cash wants to be turned into something harder to trace, and that turning leaves records of its own.
Fourth, the fraud triangle, which every student of the subject learns in the first week. Theft by a trusted person needs three things together: pressure, such as debt or a lifestyle beyond one's means; opportunity, meaning access and weak controls; and rationalisation, the story the person tells themselves. Opportunity was present in every branch by design, since the sub-postmaster handled the cash. Opportunity is the least informative of the three precisely because it is everywhere. Pressure and rationalisation have to be shown, and they usually leave evidence: arrears, county court judgments, gambling accounts, a change in behaviour noticed by staff.
A competent investigation of an honest sub-postmaster would have found none of those things. That absence would have been the most important finding in the file. In fraud work, finding no trail is itself a result. When a theft is alleged and careful tracing finds no destination for the money, no conversion, no pressure and no change in circumstances, the investigator has learned something. It does not prove innocence. It does mean the allegation has lost the one thing that should hold it up.
Paragraph 121 of Hamilton is plain about what happened instead. The court records that the Post Office had a "clear duty to investigate all reasonable lines of enquiry," and that "it does not appear that any attempt was made to investigate the assertions of SPMs that there must be a problem with Horizon." Among the submissions recorded in the judgment is that, in a case before the court, no ARQ data was obtained and there was no investigation of the integrity of the Horizon data. (ARQ data is the audit record request: the underlying transaction log held by Fujitsu.)
The evidence that should have been gathered at the start was never gathered at all.
There is a second level to this, and it is where I think the institutional failure lies.
In a single branch, a shortfall is an anomaly in that branch's figures. But the Post Office was not looking at one branch. It had hundreds of branches reporting shortfalls the sub-postmasters said they could not explain, year after year. The court found that from 2000 onwards sub-postmasters raised problems with Horizon, under both the original system and the later Horizon Online.
Step back from the individual file and look at the population, which is what anomaly detection is for. The question changes. It is no longer why is this branch short? It is why are so many branches short in ways their operators cannot account for?
Broadly, there are two kinds of answer. Either the Post Office had, by bad luck, recruited many hundreds of thieves who all stole in a similar way and all denied it in similar terms. Or something common to those branches was producing the figures. The most obvious thing they had in common was Horizon.
When the same anomaly appears across a whole population of people whose chief common factor is the system recording them, you test the system first. That is the elementary inference my discipline is built on. In a payroll review, if every employee on one site shows the same overpayment, you do not open a hundred disciplinary files. You look at the payroll run.
The Court of Appeal found that the Post Office "knew that there were serious issues about the reliability of Horizon," and that its consistent failure to be open about them "can in our view only be explained by a strong reluctance to say or do anything which might lead to other SPMs knowing about those issues." I do not speculate beyond what the court found. But I note that a reluctance to look at the population is exactly what would stop an organisation drawing the population-level inference. Each case was dealt with as if it were the only one. When looked at one at a time, a systemic fault looks like a crowd of individual dishonest people.
Something should be said about the law that made this easier, because it bears on method too.
Until 1999, section 69 of the Police and Criminal Evidence Act 1984 provided that a document produced by a computer was not admissible in a criminal trial unless it was shown that the computer had been operating properly. Section 60 of the Youth Justice and Criminal Evidence Act 1999 provided that section 69 "shall cease to have effect." What took its place was the common-law presumption that a computer was working correctly unless there was evidence to the contrary.
I am not a lawyer and I do not take a side on the drafting. As a matter of evidence, though, the effect is clear enough. A presumption of reliability does the investigator's verification for him before he has started. It turns has anyone checked? into has anyone proved it wrong? And the person best placed to prove the system wrong, its operator, holding the logs, was the prosecutor. The sub-postmaster was the person least able to do it, with nothing but a till, a safe and a conviction that the figures were wrong.
A presumption is a convenience for courts. It is no excuse for the investigator. Nothing in the law of evidence stopped anyone from obtaining the audit data, tracing the accused person's finances, or looking at how many other branches were reporting the same thing. The presumption lowered the bar for getting a case to court. It did not stop anyone doing the work.
In any report I write, I ask myself before opposing counsel does: what is the weakest point in this analysis? I should do the same here.
The objection would be that some sub-postmasters may in fact have stolen. That is true, and the Court of Appeal was careful about it. It did not quash every conviction put before it; three of the forty-two appeals failed. Theft from post offices did happen and will happen again. Nothing I have written suggests that a shortfall can never be evidence of theft.
My argument is narrower. A shortfall shown only by the system under suspicion, with no independent evidence of loss and no trace of where the money went, is not evidence of theft. It is the start of an inquiry. Where the inquiry is carried out, the dishonest are separated from the honest, because the dishonest leave trails and the honest do not. Where it is not carried out, the two cannot be told apart, and everyone is treated as the worst of them.
A second, honest limitation: I am working from the published judgments and the public record, not from the case files. I cannot say what any individual investigator knew, or what was in any particular file. I can say what the Court of Appeal found to be missing across the cases it heard, and I can say what my discipline requires. The gap between those two things is what this essay is about.
One last point, about my own profession.
In civil cases, an expert's overriding duty is to the court, not to whoever instructs or pays them. The criminal rules impose the same duty. I learned this by heart, and I think of it more often than any statute. An expert who advocates for their client destroys their own credibility, and deserves to.
In the Horizon cases the prosecutor and the owner of the system were the same body. That is a structural conflict, and an investigator in that position has a heavier duty, not a lighter one. If the evidence does not support your employer's case, you say so. That is the only kind of opinion worth giving, and the only kind worth paying for.
The gentlest way I can put the lesson is this. When a sub-postmaster said, as hundreds did, I have not taken anything, the figures are wrong, there was a simple test available. Look for the money. If it had been taken, it would have been somewhere. Nobody went to look.
Felix Aubrey Sharpley, Simulacrum · Universitas Scholarium · universitas-scholarium.org
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Written 2 October 2026.
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