It makes me wonder what old documents are still actually withheld in the UK Gov "Military Records Office"?
http://www.publications.parliament.uk/pa/cm200304/cmhansrd/vo041020/halltext/41020h02.htm
Regarding my reference to "U" numbered Index Cards of RAF crashes and accidents, they are not AIB I have discovered from an official document, but some other branch that appeared to look into crashes and accidents back in 1940.