It has been the impossible goal of British foreign policy for nearly four decades: trying to identify the so-called “moderates” in Iran’s political establishment that the West could do business with.
Back in the 1980s I remember Sir Geoffrey Howe, the then foreign secretary, telling me how he hoped to be able to secure the freedom of Western hostages held in Beirut such as Terry Waite by opening a dialogue with Hashemi Rafsanjani, then the speaker of the Iranian majlis, or parliament, who was regarded as one of the regime's leading moderates.
The initiative came to nothing, and Mr Waite and his fellow captives remained chained to their radiators in Lebanon’s Bekaa valley for another five years.
It was the same story during the Clinton administration in the1990s, when Washington thought it could defuse tensions with Iran over its pursuit of nuclear weapons by establishing a dialogue with Mohammed Khatami, Iran's so-called moderate president.
Again, the West found itself being played for a fool. Instead of engaging in meaningful dialogue with the major powers, the regime simply took advantage of their naivety to continue work on its clandestine nuclear weapons programme, with the underground Natanz nuclear enrichment facility being constructed during their period.
Now British foreign secretary Boris Johnson has found himself the latest victim of Tehran's mastery of double-dealing following his abortive visit to Tehran to secure the release of Nazanin Zaghari-Ratcliffe, the British-Iranian woman who has been jailed in Tehran's Evin prison on what are widely regarded a trumped-up spying charges.
Mr Johnson found himself obliged to visit the Iranian capital on Mrs Zaghari-Ratciffe’s behalf after he found himself embroiled in a political row in London over ill-considered remarks he made before a parliamentary select committee last month about her activities prior to her arrest and conviction in Tehran.
Mrs Zaghari-Ratcliffe’s family insist that she was simply visiting her Iranian relatives at the time of her detention, and was not involved in any untoward activities. But addressing a group of MPs, Mr Johnson suggested Mrs Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, had been involved in training Iranian journalists, an activity the regime regards as being one that is hostile to the Islamic republic.
Mr Johnson's visit to Tehran, therefore, was designed as much to repair his political reputation as to seek better diplomatic relations with the ayatollahs, and in this context the received wisdom is that Iranian foreign minister Mohammed Javed Zarif would be the best bet for making some progress on the vexed issue of Mrs Zaghari-Ratcliffe's release from prison.
As the Iranian point man on the controversial nuclear deal Iran signed with six leading world powers in 2015 to freeze its nuclear weapons programme, Mr Zarif is regarded by some Western policymakers as representing the more moderate wing of Iran’s complex religious establishment.
Many western policymakers have been seduced by the misapprehension that the American-educated Mr Zarif is an all-round moderate, someone who understands how the West works and wants to establish a constructive dialogue with the outside world.
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But as is so often the case when it comes to the West's confused dealings with Iran, this perception is little more than a mirage. Mr Zarif might well be the acceptable face of the Islamic Republic to some western policymakers, but at heart he is a fully signed up member of the Islamic revolution, and an obedient servant of the country's all-powerful Supreme Leader, Ayatollah Ali Khamenei.
This was made abundantly clear during the tortuous nuclear negotiations in Geneva where, whenever any important concession was being discussed, Mr Zarif had first to check with his masters in Iran before making an agreement.
Indeed, the fact the agreement was reached at all had more to do with the Obama administration’s desperation to secure a legacy-enhancing deal than the ayatollahs’ willingness to make any meaningful concessions, which is one of the reasons Donald Trump is so critical of what was finally agreed.
And this institutional double-dealing by the ayatollahs was evident again when Mr Johnson visited Tehran to discuss a variety of bilateral issues, including the regime’s appalling treatment of Mrs Zaghari-Ratcliffe.
While Mr Zarif dropped tantalising hints that a deal to secure her release might be possible, the cold logic is that he does not have the power to strike such a deal, even if he wanted to do.
For, as with everything else in Iran, the ultimate decision-making power rests with Mr Khamenei and the Revolutionary Guards cohorts that sustain him and the Islamic regime in power.
For Mr Johnson to secure Mrs Zaghari-Ratcliffe's release, he would have been better advised to open a negotiating channel with Qassem Suleimani, the head of the Revolutionary Guards elite Quds Force, the man widely regarded as one the country's leading power-brokers.
American and British military officers well understand the commanding influence Mr Suleimani exerts over all areas of Iranian policy-making from the malign influence he has exercised during the recent conflicts in Iraq and Afghanistan. More recently he has played a prominent role in propping up the Assad regime.
Which means that, if the British government has any hope of getting Mrs Zaghari-Ratcliffe home for Christmas with her family, then it needs to talk to real decision-makers like Mr Suleimani, and not impotent wannabees like Mr Zarif.
Con Coughlin is the Daily Telegraph’s Defence and Foreign Affairs Editor and author of “Khomeini’s Ghost”