The four IRA participants in what would become known as the 'Balcombe Street gang' were part of a six man IRA Active Service Unit (ASU) that also included Brendan Dowd and Liam Quinn. Quinn had recently shot dead police constable Stephen Tibble in London after fleeing from police officers. The flat where he was seen fleeing from was discovered to be a bomb factory used by the unit.
The Balcombe Street siege started after a chase through London, as the Metropolitan Police pursued Hugh Doherty, Joe O'Connell, Eddie Butler and Harry Duggan through the streets after they had fired gunshots through the window of Scotts Restaurant in Mount Street, Mayfair for the second time. They had previously thrown a bomb through the restaurant window on 12 November, 1975, killing one person and injuring 15 others. The Metropolitan Police Bomb Squad had detected a pattern of behaviour in the ASU, determining that they had a habit of attacking again some of the sites they had previously visited. In a scheme devised by a young Detective Sergeant, the Met flooded the streets of London with unarmed plain-clothes officers on the lookout for the ASU. The four IRA men were spotted as they slowed to a halt outside Scotts and fired from their stolen car.
Inspector John Purnell and Sergeant Phil McVeigh, on duty as part of the dragnet operation, picked up the radio call from the team in Mount Street as the stolen Cortina approached their position. With no means of transport readily available, the two unarmed officers flagged down a taxi cab and tailed the men for several miles through London, until the IRA men abandoned their vehicle. Purnell and McVeigh continued the pursuit on foot, dodging handgun fire from the IRA. Other officers joined the chase, with the four IRA men running into a block of council flats in Balcombe Street, adjacent to Marylebone rail station, triggering the six-day stand-off.
The men surrendered after several days of intense negotiations with the Metropolitan Police Bomb squad officers Detective Superintendent Peter Imbert and Detective Chief Superintendent Jim Nevill with the ASU’s leader, Joe O’Connell who went by the name of “Tom”. The other members of the ASU were named “Mick” and “Paddy”, thereby avoiding revealing to the negotiators precisely how many of them were in the living room of the flat. The resolution of the siege was a result of the combined psychological pressure exerted on the ASU by Imbert, and the deprivation tactics used on the four men. The officers also used carefully crafted misinformation, through the BBC radio news – the Police knew the ASU had a radio – to further destabilize the ASU into surrender
During their trial they instructed their lawyers to "draw attention to the fact that four totally innocent people were serving massive sentences" for three bombings in Woolwich and Guildford. Despite claims to the police that they were responsible they were never charged with these offences and the Guildford Four and Maguire Seven remained in prison. One (Giuseppe Conlon) died in jail and the rest were released fifteen years later after it was determined that their convictions were unsafe.