Her blonde hair covered with a black headscarf and puffing on a cigarette, Faye Turney enunciated each word carefully to her unseen interviewer.

Her patrol boat had “obviously” trespassed into Iranian waters, she said. But she and her fellow servicemen were being treated well by their “friendly, hospitable, thoughtful and nice” captors.

A letter apparently penned by Leading Seaman Turney, 26, in which she says she has written to the Iranian people to apologise for “apparently” entering their waters, was released simultaneously by the authorities in Tehran.

After parading the 15 British service personnel in their uniforms on television, Tehran demanded a similar apology and a formal admission of guilt from the British Government.

A few hours later, however, Iran’s leaders softened their rhetoric, announcing that British officials would be allowed to visit all the captives, and hinting that Leading Seaman Turney would be released “as soon as possible”.

In her “letter”, she wrote of her hope of being freed in time for her daughter Molly’s third birthday party. Describing herself as “well and safe”, Leading Seaman Turney wrote that she and her fellow captives were being fed three meals a day.

“The people are friendly and hospitable, very compassionate and warm. Please don’t worry about me, I am staying strong. Hopefully it won’t be long until I am home to get ready for Molly’s birthday party with a present from the Iranian people,” she wrote. She signed off her letter with 12 kisses — a detail along with all the others that will be analysed closely in London.

Her television performance, filmed in front of a floral curtain, was immediately called into question: the Foreign Office condemned the footage as unnacceptable and body language experts cast doubt on the sincerity of her words. Margaret Beckett, the Foreign Secretary, suggested that the sailor could have been coerced into making her statements.

The pictures recalled the footage of the party of eight Marines and sailors, seized by the Iranians in 2004, who were paraded blindfolded, led into a ditch and made to endure a mock execution for the cameras.

But diplomats suggested last night that the very different tenor of yesterday’s images could amount to the start of a climbdown by Iran, which had realised that it had made a mistake and was trying to find a face-saving way out of its embarrassment.

The pictures aired after Tony Blair was cheered in the Commons as he told MPs that it was time to “ratchet up” the diplomatic and international pressure on Iran. He said that Tehran had to understand its “total isolation” on this issue.

That isolation grew more apparent last night as Western and Islamic leaders rounded on Tehran. Angela Merkel, the Chancellor of Germany, said that Iran’s behaviour was “unacceptable”. Turkey and Iraq intervened publicly on the Britons’ behalf.

The gathering pressure on Iran followed the release by the Ministry of Defence of evidence that the Britons’ vessels had not violated Iranian waters when they were seized last Friday, but had instead been in Iraqi waters.

The MoD backed up its assertion by releasing a photograph of a handheld global positioning satellite device in HMS Cornwall’s Lynx helicopter as it flew over the searched merchant vessel, confirming its position. In a key disclosure Britain took the unusual step of divulging the contents of diplomatic exchanges that immediately followed the “ambush”.

Iran, it emerged, gave coordinates to British diplomats as proof that the Royal Navy had strayed into their waters. But the compass points given were actually in Iraq’s part of the Gulf, Mrs Beckett had told Manouchehr Mottaki, her Iranian counterpart, on Sunday.

The Iranians, undaunted, just changed the co-ordinates on Monday.

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