The real problem is this isn't a "right" that the postmasters could exercise. You're accused of stealing money. You're not paid very much in the first place and have only limited legal aid, and may even be in prison (on remand) already. There's no "defense team", you only have a solicitor assigned to you at the police station. You're also not a programmer. How are you going to argue that the database which shows you stole money wasn't working correctly?