Ms Truss, the first female Conservative ever to head up the Foreign Office, approached the Iran talks with two key drives, according to those in her inner circle.

One was to build up a personal relationship with the two key politicians involved. One was Mr Amir-Abdollahian. He spoke English, but often the pair talked through translators.

The other was with Sayyid Badr Albusaidi, the foreign minister of Oman, a Gulf nation that has better links with the Iranian government than most other countries.

The role of Oman as the broker and facilitator of Wednesday’s deal has been largely shielded from public view and remains shrouded in mystery.

It is clear that the Foreign Office saw Oman as critical, given its open lines to London and Tehran, in attempts to reach a deal – which explains Ms Truss’s personal engagement.

When she used her “grace and favour” estate of Chevening in December to gather a group of leaders from the Middle East, Sayyid Badr was one of half a dozen invited.

At one point, away from group discussions, Ms Truss and the Omani diplomat held a one-on-one talk in a corner of the 16th century manor house. It was an attempt to further the negotiations with Iran already under way.

The second key focus Ms Truss wanted to bring to the impasse, according to sources close to her, was “creative thinking”.

The web of sanctions imposed on Iran over the collapsed 2015 nuclear deal, which Donald Trump withdrew America from, made paying the tank debt complicated.

“Liz basically asked officials to think more creatively about how they would solve the debt problem and the detainee problem,” said a Foreign Office insider.

The intensity of the talks throughout the autumn and winter reflects the new “put in the hard yards” approach demanded by Ms Truss.

In both October and November, an experienced team of British negotiators went to Iran to try to bridge the gap between both sides. Conversations were said to have been “rigorous”.

Simon Shercliff, the UK’s ambassador to Iran since 2021, who has a national security background, played a key role in negotiations. So too had his predecessor, Rob Macaire.

‘My mummy is in prison’

The relatives of all three detainees were kept in the loop with developments, but there had been so many false dawns that their hopes were tempered by caution.

Last Christmas, the fifth she has spent with her family torn apart, Gabriella Zaghari-Ratcliffe asked her father two simple questions.