There is definately something rotten here.
Who is Harper calling after Tarrant called him? That second call is vital. And classic Siegel.
Initially I thought the kidnappers were just lucky and trying to exploit the situation.
However are we really to believe that Julyan is the man behind it all for the sake of the wife's spenditure? No way. He wouldn't have staged such a set up with the ruthless McKee & Co just for that purpose. He is not that kind of man.
It seems more logical if Tarrant staged his own son's kidnapping by giving him the plane in order to help IRA. Or that Harper is the mole. And Julyan is in any case just a stupid helper send out on an errand.
But I would probably bet on Tarrant as the inital bad guy. Otherwise the ending would be open as to Harper's further actions. Then Tarrant is also sort of forced to solve his own scheming plan. That's why he sit in the chair in Harper's office for so long after Harper refuses to give him the diamonds. And frustrates.
This still doesn't explain who Harper calls and why, however.
It's really hallucinatory this movie. The scrabble letters are in the credits only because the phone secretary with Harper calls Drabble scrabble. And all the references. What are we to make of them. Touch of Evil, Man Who Knew Too Much, James Bond, Ipcress file, Sound of Music.

It seems that way, but such big-time and careful criminals would have watched the boy's routine for ages- playing through the fields en route from school, etc, and the airfield was an abandoned local one known to the boy.