The settlement of a 40-year-old debt dating back to the time of the Shah of Iran was the key to unlocking the deal which secured the release of Nazanin Zaghari-Ratcliffe and her fellow detainees.
The payment of almost £400 million to Tehran was the final step in a carefully choreographed set of diplomatic manoeuvres which saw the British-Iranian mother board the flight back to freedom and her family.
