Hundreds of sub-postmasters were jailed or financially ruined after computers said money was missing from their branches, but the Post Office has admitted that its Horizon 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.