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 £400million 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.
It relates to an order placed by the pro-Western Shah in the 1970s for 1,500 British Chieftain tanks and 250 armoured recovery vehicles for the Iranian armed forces. An estimated £650m was paid to International Military Services (IMS), a subsidiary of the Ministry of Defence, for the equipment. But when the Shah was toppled in the Iranian revolution of 1979, Britain cancelled the order with only 185 tanks having been delivered.
Iran has been trying to get back the outstanding cash for the undelivered vehicles ever since. In 1990, the Tehran government began lengthy proceedings through the International Chamber of Commerce (ICC), which rules on trade disputes between countries, to recover the money. In 2001, the ICC found in favour of Iran, a ruling that was upheld on appeal in 2009.
