Action men: Mr Horsfall says of the film: 'What angered me most was the portrayal of the other SAS men.
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.
They all come across as grunting simple-minded men who can't string a sentence together.
'When they do speak they all seem to have Scottish accents. Those taking part were all excellent, highly trained soldiers.
The film has now been dramatised by Hollywood but has sparked outrage 'I was just so angry watching the film because it was all wrong,' Horsfall, 60, told Mail Online.
'It was so bad I had to walk away at one point.'From the supposed training missions to the actual rescue everything in the film is wrong.'Those watching it would think there were just eight people taking part.'But there were five teams of eight men who stormed the embassy but they are just left out of the film.'It is laughable and an insult to the memory of all those who took part.'The film '6 Days' stars Jamie Bell as the leader of the SAS soldiers who have to rescue 26 hostages from the Iranian Embassy in London.
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.
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.
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.