Immune from Public Interest?

Mystery from my old neck of the woods. Wang Yam, 'financial trader', on trial for the murder of Allan Chappelow, 'prize winning novelist', though who knew him when he was a reclusive resident of Downshire Hill? It's unfolding in the Central Criminal Court, and rendered all the more sinister by the slapping of Public Interest Immunity on parts of the proceedings. Public Knowledge Immunity, surely? Why has the government signed off this secrecy, usually reserved for those cases with national security complications?

http://en.wikipedia.org/wiki/Allan_Chappelow
http://www.timesonline.co.uk/tol/news/uk/crime/article3042941.ece

The Evening Standard go as far as saying Yam was a ‘low-level informant’ for MI5, which sounds plausible:
http://www.thisislondon.co.uk/news/article-23427506-details/MI5+wants+millionaire

The guardian goes for MI6/SIS:
http://www.guardian.co.uk/uk_news/story/0,,2227929,00.html

The murder was all part of an identity theft, so the story goes, although Yam's lawyer - the irrepressible Geofrrey Robertson - asks why Chappelow's passport, wallet and personal identification numbers were left in the house. And why would Yam then be so foolish to try and access Chapelow’s accounts in the full glare of CCTV? Not one piece of forensic evidence links Yam to the murder.

Then there’s the question of the fire that broke out nine days after the discovery of the body, destroying 35% of the ground floor. How does that happen? Police are remaining 'tight lipped', according to the Camden New Journal:

http://www.thecnj.co.uk/camden/062906/news062906_04.htm

Doubtless it will all prove far less interesting than it seems. But the need for PII is intriguing. How many slightly higher-level informants find the criminal justice system bent in their favour to save blushes all round?

I remember passing Chappelow's derelict home as a child, going to the Heath, and even, possibly, seeing the 'reclusive writer' himself in Swiss Cottage library, a haunt we apparently shared.