I believe there is a direct correlation between the number of people required to pull it off and the length of time it could remain secret...
Diana: 4-man squad, commanding officer, couple of logistics people getting equipment into France, at least 4 MOD people who knew special ops team in...
I don't accept any of the grand conspiracy theories banded around.
Ultimately, unless only 1 or 2 people were involved, no conspiracy would remain a secret for any significant amount of time - somebody would tell a spouse who then becomes an ex or make a deathbed confession or would generally...