For Nazanin Zaghari-Ratcliffe, the end of her sentence may not mean freedom

After five years of imprisonment and house arrest, Nazanin Zaghari-Ratcliffe must have felt a surge of elation when Iranian prison officials finally removed her ankle tag on Sunday. But relief will be tempered by the knowledge that the end of her prison sentence does not necessarily mean her freedom.

The blunt truth, her family fear, is that she is one of several British-Iranian hostages being held for ransom. And that ransom has still not been paid.

Neither government wants to admit to anything so shabby in public. But over the past five years, Iranian officials have explicitly told Mrs Zaghari-Ratcliffe and her family in Iran that her detention is linked to a multi-million-pound debt the UK owes Iran for failing to deliver on a 1970s arms deal.

Richard Ratcliffe, her husband, says that has been confirmed to him by international sources he would rather not name. And that is why he is guarded about what happens next: He does not believe that after five long years, the Iranian Revolutionary Guards Corps are willing to give up their hostage until they see the colour of the UK’s money.

The dispute goes back to the 1970s, when the Shah of Iran ordered over 1,000 Chieftain tanks and armoured vehicles from International Military Services (IMS), a UK Ministry of Defence-owned export company. After the Shah’s government was overthrown in the 1979 revolution, the deal fell apart - but Britain did not return the advance payment it had already received.

In 2008, an international arbitration court ruled in the Iranian Ministry of Defence's favour and said the UK should pay the debt. But lawyers for IMS, which now exists only as a vehicle to settle this dispute, have fought an epic legal battle over the size of the payment and whether it should pay interest.

The next hearing of the dispute is due in a London court on April 20. It will be relatively simple to keep Mrs Zaghari Ratcliffe in Iran until then - or even beyond - if her captors wish to. Most easily, officials could simply refuse to give back her passport. That would leave her ostensibly free, but unable to leave the country to rejoin her husband and daughter. It is a tactic that other prisoners have faced in the past.

More menacing is the threat of a second criminal case. In September, she was told she would face fresh charges of "propaganda against the regime” - allegations that may be linked to remarks Boris Johnson made when he was Foreign Secretary.

Her family say there is no new evidence that would justify a fresh prosecution. But she has been summoned to court again next Sunday, raising fears she could be hit with a new conviction. British officials have always down-played any supposed link between the two cases, briefing journalists who raise the issue that it would be irresponsible to draw any such connection.

But there has recently been a change of tone in Whitehall. Mr Ratcliffe told The Telegraph last week that Dominic Raab, the Foreign Secretary, did not dispute the existence of a link at a recent meeting about his wife’s case.

Don't expect Britain or Iran to ever acknowledge such a deal. But the April court hearing in London could mark the moment when an understanding is reached.