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//I don't see how it caused the car crash.//
It didn’t ‘cause’ the car crash. Tina Brown is saying that had Diana not dismissed her security people as a result of losing confidence in their loyalty, she wouldn’t have been in that car. They would have been around to stop her.
Whether the documents were used, or whether Bashir had them ready to use if they were needed, as per Ellipsis’ post, the question of whether they should have existed in the first place should be asked. As former chairman of the BBC, Lord Grade says, "There is a very dark cloud hanging over BBC journalism and it is a ‘very, very serious matter’. It’s not good enough to say ‘She’s dead - ignore it’, which is why the BBC is launching an investigation.