Iran now claims that it is enriching uranium on an “industrial scale”, but there is not much reason to believe that this is yet true, unless it has in mind a cottage industry.

Of course, that is no reassurance about Iran’s intentions. We should assume that it wants to put itself within easy reach of nuclear weapons, for all its assertions to the contrary — and President Ahmadinejad all but said that on Monday. But the eruption of propaganda from Tehran — including the showy release of the captured British sailors and Marines — does not conceal either the weaknesses of the regime or the signs that sanctions are working. For all the excoriation in Britain of the decision to allow the crew to talk to the media, their accounts will add to the pressure on Iran.

The new phase of Iranian belligerence began three weeks ago when the United Nations Security Council passed a new resolution to ratchet up sanctions. Like the first resolution in December, the solid support for this move (after noisy misgivings by South Africa) appeared to take Iran by surprise. It responded with the seizure of the British boat, withdrawing cooperation from the International Atomic Energy Agency, the UN watchdog, and, this week, saying that it was ready to start uranium enrichment, the most controversial part of its nuclear work.

It may be ready in the sense of wanting to start, but that does not mean that it can. This is a country, after all, that has been prevented by sanctions and lack of expertise from building refineries to turn its own oil into petrol. It has been forced to spend the bounty from high oil prices on subsidising imported petrol to fend off protests that might threaten the regime. Nor, come to that, has Iran been able to make spare parts for its national airline, which has a safety record so poor that buying a ticket carries a significant risk of death.

Mark Fitzpatrick, a nuclear proliferation analyst at the International Institute for Strategic Studies, has argued steadily that Iran is struggling to master uranium enrichment, the trickiest step in making fuel for power stations or fissile material for bombs. Ali Larijani, the chief Iranian negotiator, said this week that Iran had begun feeding uranium gas into 3,000 centrifuges. But although this is about ten times the number of centrifuges that Iran was previously thought to have, it does not mean that they are working properly; even its pilot plant has been severely delayed.

True, it is unfortunate that Iran has shut the doors to IAEA inspectors. Dissidents originally tipped off the world about Iran’s covert work but since then details, if incomplete, have come from the IAEA. Yet even if that intelligence will soon be out of date, at the moment it confirms the picture of Iran’s shaky mastery of the technology.

Threats by Iran this week will only increase the willingness of countries to close ranks against it. So, too, will its behaviour over the British captives, and so may their personal accounts, for all the dismay they have triggered in Britain. The Royal Navy and the Army should be perturbed that Faye Turney and 14 young servicemen were so ill-prepared for handling themselves in captivity that they lent themselves so quickly to supporting Iranian claims about the location of their boat. But the national storm about their accounts has accused them of giving Iran a propaganda triumph — and in the search for pessimism with which to berate them, that is too neat.

It springs from a fantasy that image can be precisely controlled, untrue even in the historic military encounters that have been recited this week with such nostalgia. The accounts of the detention will have left many people — not just in Britain — with a lasting impression of Iran’s malign behaviour, regardless of what they suggest about the Armed Forces. When the Security Council next comes to judge Iran’s compliance, in five weeks’ time, that malevolence is bound to dominate the impression.

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