I agree with Nessie on the issue of charging perverting the course of justice v fraud.
IANAL, but as I understand UK law, there are ten elements of a fraud charge, all of which must be proved for a fraud charge to succeed....
-False representation
-Untrue or Misleading statements or behaviour
-Dishonesty
-Gain or Loss
-A Failure to Disclose Information
-Establishing a Legal Duty
-Abuse of Position
-Possession or Control
-Show articles and materials of the fraud
-Obtaining of services or benefits
It will require a crapload of detailed documentation and complex evidence to prove a such a wide ranging, multi-layered fraud case such as this, and will take years to put a watertight case together.
On the other hand, proving a charge of perverting the course of justice is relatively straightforward, and there is ample evidence that has already been established as fact.
Perverting the course of justice can be ANY of the following
-Intimidating or interfering with a case witness, juror or judge
-The disposal, or fabricating, of evidence
-Falsely accusing someone of a crime, resulting in their arrest.
Only one of these has to be shown to be true, but it can be easily shown that the Post Office committed all three on many occasions.
Intimidating or interfering with a case witness, juror or judge
The Post Office lied to the sub-postmasters. They told each one of them that they were the only one who was having difficulty with Horizon. In effect, they isolated them, and threatened them with prosecution, and when some postmasters stood their ground, they did prosecute them.
The disposal, or fabricating, of evidence
The Post Office and Fujitsu knew there were serious issues with the reliability of Horizon, and they went to great lengths to cover this up, even to the point of destroying evidence by shredding documents and deleting digital files that might reveal what was going on.
Falsely accusing someone of a crime, resulting in their arrest.
This is what they did, routinely to over 700 sub-postmasters. They actively hid the evidence that the fault lay with the Horizon software and not with the sub-postmasters, and then they false prosecuted the sub postmasters.
Also, there were multiple people who had meetings in which they resolved to cover up what happened. Meeting from which orders were given to Post Office employees to destroy documents and evidence. Those meetings are evidence that multiple people who worked together to cover it up - so could all be charged with conspiracy to pervert the course of justice.
There is a well-understood principle in prosecution of criminal acts... charge those crimes that are easiest to prove rather that those that are difficult to prove. Perverting the course of justice will be a relative doddle co prove compared with fraud.