Hundreds of postmasters were jailed or financially ruined after a computer system said money was missing from their branches. Now the Post Office has admitted that its Horizon computer system can make mistakes. But when did senior managers find this out, and did they continue to prosecute postmasters for stealing when they knew technology could be to blame? Reporter Nick Wallis investigates what could be Britain’s biggest ever miscarriage of justice scandal and uncovers evidence of a cover-up at the Post Office.