POBJOY, if you want to know what happened and when it happened then I suggest you read David Hill's book above. As to why it happened, why RAF VSOs chose to order that mandated regulations be suborned, ie disregarded but signed off as if complied with, and why those airworthiness engineers who did not obey such illegal orders but strove to carry out their mandated duties were acted against, dismissed, and replaced with untrained and unskilled administrators, you would have to ask those same VSOs.
That is the point, no one has interviewed them, charged them, or taken any action whatsoever in their regard, despite evidence being presented to the civil police, the military police, and the Nimrod and Chinook enquiries. David Hill suggests possible reasons, but that is as close as we get to answering your:-
The question has to be WHY and for what reason would any sane person seek to diminish the quality of airworthiness at any level of aviation, and especially that involving complex machines (RAF) and the training of minors (ATC).