The peaceful London protest that became a day of bloody infamy

It began with what should have been a peaceful demonstration watched by benevolent London bobbies on 17 April 1984. Outside a building which Colonel Gaddafi insisted on calling the Libyan People's Bureau, in St James's Square off Pall Mall, a little group of Libyan exiles had gathered to protest at the hanging of two Tripoli University students.

The police did not expect the protesters to cause any trouble, and what should have eased their task was that it was a holiday in Libya, so most of the staff of the embassy, or People's Bureau, were not at work that day. Neither the police nor the demonstrators reckoned on the fanaticism of some of the Gaddafi loyalists who were inside the building, staring resentfully out at their fellow countrymen shouting slogans against their leader.

One of them, in an upstairs room, raised an automatic weapon and raked the crowd with bullets, hitting 11 of the protesters. All, mercifully, survived, though five were seriously injured.

But one bullet hit a 25-year-old police constable, Yvonne Fletcher, who by rights should not have been in the police force at all because she was just 5ft 4in tall. But somehow she had talked her way into getting a job and was engaged to a fellow officer, who was standing nearby and saw her die.

It is the only instance in living memory in which a British police officer has been murdered in the line of duty and the culprit has got clean away – which is why Yvonne Fletcher is the only murdered officer whose name can be instantly recalled by a very large number of people, although she has been dead for almost 27 years.

Though the Libyans refused to call their premises an "embassy", its staff enjoyed all the privileges of accredited diplomats, which meant that the police were not allowed to go into the building to arrest the killer. All they could do was surround it, to stop him getting out.

The reaction in Tripoli was instant. Troops encircled the British embassy, trapping 20 people inside, and Colonel Gaddafi vowed that if their bureau was stormed "an act of this magnitude will not go unanswered by the Libyan people".

The stand-off lasted several days, while the British authorities sought Libya's permission for detectives to enter the building. They kept in telephone contact with staff inside, and took them supplies of food, drink and cigarettes, while armed police trained their weapons on the building, day and night. In Tripoli, the British embassy was under a similar siege.

After six days of tense and ultimately pointless negotiations, the British government broke off diplomatic relations with Libya, ordered the staff from the Tripoli embassy home, and gave Libya's diplomatic staff one week to leave the country. The implication was that after a week they would lose their immunity and the police would be free to do what they could to identify and arrest the gunman.

Even as they left, they and their baggage were accorded diplomatic status, which meant that on 27 April police had to stand back, under the watchful gaze of diplomats from Turkey, Saudi Arabia and Syria, as dozens of bags were removed from the building – knowing that one of those bags held the gun that had killed Yvonne Fletcher. The next day, 30 people trooped out of the People's Bureau and boarded a plane for Tripoli.

The police believed they already knew the killer's identity. Using monitoring equipment, they had overheard a heated argument inside the building during which the gunman's name was mentioned. All 30 occupants had to give their names as they left. Only one matched.

He was said to be a man with dark hair, in his early 30s, a description that fits Omar Ahmed Sodani, who has always maintained his innocence. Though it was reported that Fletcher's killer was executed on arrival in Libya, the truth appears to be that he was given a hero's welcome.

Diplomatic relations between Britain and Libya were severed for 15 years, until July 1999, after Gaddafi had agreed to hand over the two Libyans accused of the Lockerbie bombing. After that, detectives from the Metropolitan Police made several visits to Libya in the hope of cracking the case, but without success.