Post Office trial throws up known problem with Horizon system

Evidence presented in the High Court has revealed a previously undisclosed problem with the Horizon computer system at the centre of a decade long dispute between the Post Office and a group of subpostmasters.

Alleged fallibilities of the Horizon system, which has been used in thousands of post offices since its introduction in 1999/2000, are at the centre of the group litigation order being heard.

More than 500 former subpostmasters are seeking damages for the suffering they experienced after some were heavily fined, had to pay back thousands of pounds in shortfalls, and in some cases were sent to prison because of unexplained discrepancies in accounts. They blame the Horizon system for the errors (see timeline below for all Computer Weekly coverage).

In 2009, Computer Weekly revealed the stories of some subpostmasters, who had received heavy fines and even jail terms for alleged false accounting, which they blamed on Horizon.

Leading up to the current trial, the Post Office reaffirmed its confidence in the Horizon system in a statement to Computer Weekly. “We have confidence in the Horizon system, which is robust, reliable and used across 11,500 branches by postmasters, agents and their many thousands of staff to process millions of transactions successfully every day, including on behalf of the UK’s high street banks,” it said.

The trial, which began on 7 November, is focused on the contractual relationship between subpostmasters and the Post Office. Horizon will be the focus of a further trial in March next year.

But in documents referred to in court and written about by broadcast journalist Nick Wallis in his Post Office trial blog, evidence of a known problem with Horizon is described by the Post Office and its IT partner Fujitsu.

A Post Office internal memo from August 2010, referred to in court and entitled Receipts payments mismatch issue memo day 1 OP_0008387, reveals that discrepancies showing at the Horizon counter disappear when the branch follows certain process steps, but will still show within the back-end branch account.

Only a small number of branches were known to have been affected at the time, according to the memo. “This is currently impacting circa 40 branches since migration onto Horizon Online, with an overall cash value of circa £20k loss,” the memo said. “This issue will only occur if a branch cancels the completion of the trading period, but within the same session continues to roll into a new balance period.”

The memo revealed that the Post Office had not communicated with branches affected at the time and did not believe they were “exploiting this bug intentionally”.

“The problem occurs as part of the process when moving discrepancies on the Horizon system into local suspense,” it said. “When discrepancies are found during stock unit rollover into a new transaction period, then the user is asked if the discrepancy should be moved to local suspense.

“If the branch presses cancel at this point, the discrepancy is zeroed on the Horizon system.”

But the document said this is not fed into Post Office systems, which still show a discrepancy while the branch system does not.