Hamilton & Ors v Post Office Ltd

Where a prosecution depends upon computer-generated accounting data, the prosecutor must investigate reasonable indications that the data may be unreliable and disclose material capable of undermining its case. Systemic failures to do so may make a fair trial impossible and constitute category 1 abuse of process.

The same misconduct may also amount to category 2 abuse where prosecution would affront the conscience of the court and undermine public confidence in criminal justice. A guilty plea does not prevent a conviction being quashed where it was founded upon the non-disclosure. The decisive question remains whether the reliability of the data was essential to the prosecution.

Forty-two former sub-postmasters, managers and counter assistants appealed following references by the Criminal Cases Review Commission. They had been convicted between 2003 and 2013 of theft, fraud or false accounting in prosecutions brought by the Post Office.

The appeals followed factual findings in the High Court that the Horizon accounting system contained bugs, errors and defects capable of causing apparent branch shortfalls: Common Issues [2019] EWHC 606 (QB) and Horizon Issues [2019] EWHC 3408 (QB).

The central question was whether the prosecutions were abuses of process because Horizon reliability was essential, POL had failed to investigate and disclose relevant material, and the resulting convictions were unsafe.

1. Thirty-nine appeals were allowed and the convictions quashed. Their prosecutions were both category 1 and category 2 abuses of process. The appeals of Wendy Cousins, Stanley Fell and Neelam Hussain were dismissed.

2. In the Horizon cases, the court accepted and adopted the High Court findings that Horizon had significant and persistent defects capable of producing apparent shortfalls. There was no independent evidence of an actual shortfall. POL nevertheless prosecuted on the asserted reliability of Horizon while failing to investigate reasonable lines of enquiry or disclose material required by Criminal Procedure and Investigations Act 1996.

3. Applying R v Maxwell [2010] UKSC 48, those failures made a fair trial impossible. They went to the foundation of the charges: without reliable Horizon data, there was no proper basis to prove missing cash or stock. The guilty pleas were founded on the same failures of investigation and disclosure and did not prevent the convictions being unsafe.

4. The same systemic conduct also established category 2 abuse. POL knew of serious Horizon concerns, consistently maintained that the system was reliable, and effectively required accused persons to disprove an apparent shortfall. Its approach prioritised its own interests over its prosecutorial duties. The gravity of that conduct, the severe consequences for the appellants, and the direct implication of the courts meant that the public interest in prosecution was heavily outweighed by the need to preserve confidence in criminal justice.

5. Horizon data was not essential in the three dismissed appeals. The cases against Mrs Cousins and Mr Fell rested on evidence independent of Horizon reliability. Ms Hussain admitted the material Horizon entries and there was compelling independent circumstantial evidence of theft. No category of abuse was established in those cases.

• Court of Appeal (Criminal Division): On references by the Criminal Cases Review Commission, the court allowed 39 appeals on both category 1 and category 2 abuse grounds, quashing the convictions. It dismissed the appeals of Wendy Cousins, Stanley Fell and Neelam Hussain.
• Crown Court: The appellants had been convicted, after pleas or trials in various Crown Courts, of offences including theft, fraud and false accounting.