The BOI did interview the witnesses, once the CM had completed which was well over a year since the accident by this stage. The BOI were as surprised by the CM investigation as anyone and there was little or no interaction between the BOI and the police.
Once the BOI had been reconvened, they had the CM evidence but chose to interview the witnesses themselves in order to gain first-hand accounts.
As for my 'inside knowledge', if refer you to my original post.
As for 'mistake', it is my opinion that the decision to go to CM so early was a mistake. We are still living with that decision and it is hurting a controller as well as the RAF as a whole in a time when we need the support of the public more than ever.