I agree that the public record must not include any detail of the proceedings of the meeting, but how do we add those details to the official records of the council? In your legal action example, a decision might have been taken to incur significant expenditure and in the future, that decision may be subject to scrutiny. Evidence will be needed to explain how and why the decision was made.
On the only occasion I have been involved in a partially closed meeting, I took minutes of that agenda item in the usual way, but published them in a separate document that was not included in the public record. I now have a dedicated file for confidential minutes, although it remains light on content! This is an old trick I picked up in school governor clerkship, which may or may not be relevant to local councils.