Has anybody been involved in, any interest in, or any knowledge of the balances in this area?

PII is where the public interest is believed to be served by maintaining the confidentiality/secrecy of particular articles or documents.  It is commonly used to protect persons in vulnerable circumstances.

Where a data subject requests access they have an entitlement which "should prevail over any provisions to the contrary contained in any other acts statutory instruments or rules of law." (The Guidelines. Third Series. November 1994. Section 2.38.)  This gives rise to the circumstance that where a claim of PII is made for a document(s) and a subject access request is made, the reply must contain the document(s) covered by PII. 

The existence of PII appears to be to protect individuals.  Subject Access does not allow access to data relating to other individuals without their permission, and as such PII appears to be incorporated within the data protection regime.  e.g. A person is protected within the subject access regime because their information cannot be disclosed to others without their permission.