In the past 48 hours Britain has increased the public political pressure placed on Iran over its de facto kidnap of Royal Navy sailors and Royal Marine personnel, and that enhanced effort may be about to bring results. What the Prime Minister described on Tuesday as a “different phase” in this affair stands in welcome contrast to an initial response from sections of the Foreign Office that smacked of treating the Iranians as excitable foreigners who would come to their senses if allowed the chance to calm down, rather than as the calculating actors that they are. Tony Blair condemned this seizure at Prime Ministers’ Questions as “completely unacceptable, wrong and illegal”. He will need to carry on being as blunt if this incident is to be resolved in a timely fashion.

There were indications yesterday that parts of the Iranian Government were beginning to feel awkward about their official posture. Their Foreign Minister was at pains to insist that Faye Turney, in particular, would be released as soon as possible. He also started to recast Tehran’s initial rhetoric that these personnel had been involved in an aggressive deed by conceding that any “violation” could have been accidental.

Iran’s awareness that its reputation is at stake was illustrated in the television pictures of the captives that it finally allowed to be aired yesterday. When eight British servicemen were taken in a not dissimilar situation in 2004 they were paraded, blindfold, marching through the desert in a knowingly provocative fashion before they were handed over soon after. The images offered this time were more conciliatory in their character. They were obviously intended to make Iran’s claims that its captives were “unharmed” and being “treated well” seem credible. The misuse of Ms Turney, nonetheless, was intensely cynical and will prompt condemnation well beyond this country. Her letter and television interview were obviously not voluntary in their nature and they constitute a somewhat desperate attempt to shore up Tehran’s standing.

It is not surprising that Iran has become more defensive in its approach to this confrontation. The detailed coordinates that have been released by the Deputy Chief of the Defence Staff, Vice-Admiral Charles Style, provide compelling proof that the British sailors and Marines were indeed conducting a wholly routine inspection in an area that is indisputably inside Iraqi waters. His further riposte that the Iranians had altered their estimate of where the British craft had been snatched, after the first set of coordinates that they produced had been shown to be outside of their waters, reinforces this evidence. It should not, however, have taken five days for this information to be placed into the public domain.

The reality is that Britain is dealing with an erratic regime. It is internally divided as to how much it is willing to back its firebrand President. Iran is more than capable of deliberately inconsistent, even incoherent, behaviour. The trait that links all the factions in Tehran, be they the “moderates” or “radicals”, is a dismissive instinct towards opponents who they believe have demonstrated weakness. The belated robustness exhibited by ministers in the past two days may have convinced those who hold sway in Tehran that a prolonged hostage crisis would do more damage to Iran than it would inflict on Britain. If that sentiment becomes dominant, then Iran will almost certainly cut its losses from this episode. It has taken a stronger British stance to secure progress so far. That approach must not slacken.