senior management enforcers.
There you go then
A very basic starter for ten: many of the subPMs were told, upon their disputing the shortfalls - "you're the only one".
Who told them that? Shouldn't be hard to find out.
Then, question them: "When you told X that no-one else had that issue, did you believe or know that to be untrue?"
Then, find out who instructed them to lie (to the subPM).
And watch the dominoes fall. Up to and including the CEO, if necessary.
Everyone has things that they don't spot, or deal with correctly, at work. It's human nature.
But we're not talking about some auditor within the PO missing that a subPM has been helping themselves to the pick n' mix.
A CEO cannot reasonably be held accountable for that.
But,
This is hundreds of significant "fraud" cases, amounting to millions of pounds.
At this scale, the CEO can't plead that they were unaware, surely?
Which leaves misled, incompetent, or complicit.
IMHO.