It is hardly 'secret information' and it is not only personal knowledge or experience of the press or 'writers' using only a part of the 'story', based on an unnamed source.
Look at this film with a little logic - HM is supposedly up in the hills, her vehicle has broken down, she wades up a bank and then sits crying until she notices a 14 pointer, which she shoo's away. How romantic... how humane.... then apply logic. If she was alone, what insider could know what happened and if HM had broken down on the estate anywhere remotely close to the guns, the first thing anyone would have done, was to notify the Gillies out with the shooting party, to ensure that no accidents could happen!
Look at the scenes of Charles telling his 'man' to ring Blair, the phone calls that were supposed to have happened, who said what, where and when. Logic says that IF any of that happened, everyone concerned would have ensured they could not be overheard.
In Campbells book and on all the interviews he has done to promote it, he has stated unequivocally, that it was not (as portrayed in the film and many press reports) he who coined the pp phrase.
Read any statements that have been put out by celebrities, to refute stories coming from unnamed/close friends/sources.