I think the article suffers a little from a confusion of terminology and the common misconceptions about the role of the AAIB (or AIB in those days) in a military accident investigation; where they don't so much investigate but examine part of the evidence. Any attempt at investigation is severely restricted by not being permitted access to key data, which means they cannot focus correctly or swiftly on areas of obvious interest (such as the aircraft not being airworthy). Also, their final reports are heavily edited and very often bear no relation to the Inspectors' reports. Chinook ZA721 in 1987 is the obvious example, where the AAIB almost immediately produced photographic evidence of most likely cause, and MoD buried it and said cause not determined.