Private meetings are fine. No issue there. The judge however, erred. He should've never allowed the disclosure to not be disclosed. You can redact information that is harmful to the state, or is not relevant to the case, but in this case, the fact that the system that was being used as the basis of detection of fraud, was in itself being audited, should've meant that it was to be included and disclosed to defense.
In my non-lawyer opinion, the judge is the one that erred, and it is compounded that this matter came up in a private meeting without defense, thereby supporting the idea that private meetings should never be had, when there are always exceptions for them.
Perhaps in the UK, but in the US a private meeting is not fine. The term is ex parte and in most states if it isn't outright banned it's highly discouraged.
Pretty much the only time it's allowed is when there's classified information in the mix, and for that there's a huge process beforehand to make sure the defense has access to enough information to make their defense.
This article presents a very odd version of what happened. Simon Clarke was the lawyer that actually stopped the Post Office prosecutions as soon as he got involved.
In this case he went to the judge and said he was aware of the existence of a report that needed to be disclosed, but it could not be given to anyone (including Simon Clarke) until the report had been presented to parliament. The judge granted the exemption for 8 weeks, after which the report was provided to the defense.
In my non-lawyer opinion, the judge is the one that erred, and it is compounded that this matter came up in a private meeting without defense, thereby supporting the idea that private meetings should never be had, when there are always exceptions for them.
So yes, this looks really bad all around.