PJ,
In accordance with MoD rules for Boards of Inquiry (BoI) into aircraft accidents, in order to pronounce negligence on the part of a crew, there must be absolutely no doubt whatsoever as to the cause. This was the original finding of the BoI concerned, which was subsequently overturned by the two Air Marshals who convened the BoI: from the outset, there was a very significent majority of those knowledgeable in these matters which asserted that they were very wrong to do so. The MoD Independent Review has upheld this view, which once and for all concludes the issue.
Hope that, in a nutshell, helps. The Government's statements later today should provide more substance.