The party of eight sailors and seven Marines were already exhausted when they set off to inspect a suspicious Indian vessel in the Gulf on the morning of Friday, March 23.

They had just completed a draining day visiting local fishing craft to hunt for drugs and weapons in the perilous waters near the Iran-Iraq border.

Arthur Batchelor, the baby of the group at 20, had e-mailed his girlfriend the previous night: “Sorry I never wrote back sooner, was kinda busy, 17-hour days yesterday but today will be better . . . ”

Before going to bed Dean Harris, a Marine, had checked his messages on Myspace and e-mailed a friend: “Let’s go away for a lads’ holiday when I get back.” He did not know Iran’s Revolutionary Guards already had a more arduous foreign break planned for him.

The party set out in two rigid inflatable boats (RIBs) for what seemed like a routine mission, the 67th such boarding by the Royal Navy since early March. Britain is leading the patrolling of the waters on behalf of the United Nations. The coxswain of one of the boats was Faye Turney, mother of three-year-old Molly and a Wren proud to say she could do her job as well as any man.

At 7.39am, the team boarded the vessel. A Lynx helicopter overhead saw that the master was friendly and returned to HMS Cornwall. The patrol’s only link to the ship was via a satellite device beaming coordinates. At 9.10am, as the boarding was completed, the signal went dead. By the time the helicopter was airborne all the crew could see was the two little boats being escorted in Iranian waters by Revolutionary Guard patrols.

The British, lightly armed with rifles and pistols, were hopelessly outgunned and had no choice but to go quietly. One burst of machinegun fire from the Iranians and the inflatables would have been sunk.

Iran claimed it had detained the Royal Navy for trespassing in its waters.

Britain began a round of robust but discreet diplomatic contacts. Iran triumphantly flourished a compass position to prove the naval crew had crossed the unmarked sea border. Margaret Beckett, the Foreign Secretary, pointed out in a phone conversation with the Iranian Foreign Minister that the pinpointed spot was outside Iran’s area of the Gulf. Iran responded by changing the coordinates.

On Wednesday, March 28, a frustrated Tony Blair took the gloves off. Adopting loudhailer diplomacy, the Foreign and Commonwealth Office risked Iran’s wrath by disclosing the humiliating error over the muddled compass position.

The Ministry of Defence held a media briefing, releasing a photograph of the boarded Indian vessel still — according to British officials — anchored in the same spot in the Gulf. A satellite positioning device could be seen held by a man on board a helicopter hovering over the location, showing that it was 1.7 nautical miles inside the Iraqi part of the Gulf.

Threatening to ratchet up the pressure, the Prime Minister sought the support of international institutions to isolate Iran.

This proved to be a tightrope walk. The United Nations Security Council expressed “grave concern” but watered down a British draft which called for the detainees’ immediate release and would have acknowledged that they were seized outside Iran’s waters.

President Putin of Russia twisted the knife, cynically suggesting a UN investigation into where the Navy had really been — a bureaucratic move that could have lengthened their detention. European Union leaders volunteered outraged noises. They blanched at immediate economic sanctions suggested by Britain. But the risk of offending the bloc which represents Iran’s largest trading partner began to concentrate minds in Tehran.

Tehran, meanwhile, played its trump card, exploiting the detainees by releasing a series of videotaped confessions. Leading Seaman Turney was the star of the show. Her blonde hair covered by a headscarf, in keeping with Muslim tradition, she admitting entering Iranian waters.

The last time Iran seized a British boarding party near the entrance to the Gulf in 2004, it paraded them blindfolded. This time, using subtle propaganda, it showed the smiling Britons pampered with cigarettes, kebabs, nuts, deep Persian carpets and games of chess.

A series of letters from Leading Seaman Turney hammered home the point: how much kinder than the way the coalition treats its prisoners at, for instance, Abu Ghraib, she said.

The written and verbal messages from the British were obviously dictated by the Revolutionary Guards. The detainees let the outside world know they were parroting from a script by failing to correct the Iranians’ grammar, spelling and technical errors.

Iran parodied the Ministry of Defence press conference by putting its own stick-wielding uniformed man in front of a map, pointing at coordinates to make its case.

British politicians could only say how more and more furious they felt. But the heat was running out of the dispute. The Foreign Office ceased to provide the media with a running commentary on negotiations.

Although President Bush said that he, like Mr Blair, opposed any “quid pro quos”, Iran began to enjoy some pleasant surprises. An Iranian diplomat who vanished after being kidnapped in Baghdad by men wearing Iraqi uniforms was suddenly found, freed and returned to Tehran.

A news agency reported that Iran was being granted access to five Iranians detained for three months by America after being removed from a consulate in Iraq. The Iranian new year is a traditional time for prisoners to be granted amnesty.

With the eyes of the world on him, in his first press conference of the year 1386, President Ahmadinejad seized a golden opportunity to appear magnanimous while scolding the West. Like the maker of a B-movie cliffhanger, he built up the suspense by first pinning medals on the “courageous” men who seized the Britons “violating” Iran. Then he announced that the British could go free.

What he said

“ How can you justify seeing a mother away from her home, her children? Why don’t they respect family values in the West?”

“I didn’t change my decision suddenly. From the beginning, I didn’t want to have any confrontation. We wanted our rights and we really didn’t want to have any confrontation. The British Government behaved badly and it took longer”

“Nothing specifically has been done by the United Kingdom. The UK Government has sent a note, a memo, and in that they mentioned that incident would not be repeated. Of course, that decision that we are going to release the 15 British sailors is not related to that letter, and it was a present from the Iranian people to the British people”

“We are a peaceful people, we want to have peace and security — we want peace and security for all people”

— President Ahmadinejad

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