Maybe I'm being naive here but I would have expected that any solicitor or barrister, who was representing the accused in Court, to have raised the issue of lack of disclosure. The Judge could then have ordered the PO to provide full disclosure.
As said above, if the Post Office said there was nothing to disclose, that would be the end of the matter.
If it was a Police investigation, they’d have got the lot, but due to it being the Post Office Investigations, they undoubtedly thought they could do what they wanted, as they didn’t play by the same rules as everyone else and didn’t see the need to.