The siege began when a group of six gunmen stormed into the Iranian embassy in South Kensington, London, on April 30th 1980.
They took mostly embassy staff hostage but among them was BBC sound recordist Sim Harris who was at the embassy to get a visa.
PC Trevor Locke, a member of the diplomatic protection squad and on duty at the building, was also among those held at gunpoint.
The terrorists demanded the release of prisoners in Khuzestan Province in southern Iran during a series of tense negotiations with the police who sealed off the embassy and surrounding streets. The gunmen also demanded safe passage out of the Britain.
The veteran was among those who took part in the famous SAS raid of the Iranian embassy in 1980
After becoming frustrated with the lack of progress over five days they shot one of the hostages and threw his body out of the embassy.
The death was the signal for the SAS – motto Who Dares Wins – to be sent in.
The SAS teams, who had arrived secretly in London from their Hereford base, were given permission by Prime Minister Margaret Thatcher to carry out a rescue mission given the code name Operation Nimrod.
On the evening of May 5th TV news cut into normal programming to broadcast the beginning of the end of the siege as the SAS soldiers all clad in black abseiled down the front of the building.
Millions watched in awe as they tossed stun grenades into the building to begin their assault.
BBC cameraman Sim Harris was captured scurrying to safety, jumping over a balcony, while smoke billowed from the building after a curtain caught fire.
What was unseen by cameras were the other teams – including Horsfall and Firmin – sweeping through the building in a race to free the hostages before they were shot.
The raid lasted just 17 minutes and all but one of the six gunmen were shot and killed.
A second hostage was shot dead by the gunmen and two others seriously wounded.
The televised raid was the first time the British public had ever seen the Special Forces soldiers in action and elevated them to superstar status.
Fawsi Najad, the surviving gunman, was jailed for life and freed in 2008 after serving 27 years. He was granted leave to remain in the UK.