Peter Imbert, later Commissioner of the Metropolitan Police, was the chief police negotiator. Scotland Yard refused, creating a six-day standoff between the men and the police. The men declared that they were members of the IRA and demanded a plane to fly both them and their hostages to Ireland. The four men went to 22b Balcombe Street in Marylebone, taking its two residents, middle-aged married couple John and Sheila Matthews, hostage in their front room. Several other police officers were also decorated. Purnell was subsequently awarded the George Medal for his bravery. Other officers joined the chase, with the four IRA men running into a block of council flats in Balcombe Street, adjacent to Marylebone station, triggering the six-day standoff. Purnell and McVeigh, unarmed, continued the pursuit on foot despite handgun fire from the gang.

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. 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 Ford Cortina approached their position. The four IRA men were spotted as they slowed to a halt outside Scott's and fired from their stolen car. In a scheme devised by a young detective sergeant, the Met flooded the streets of London with plainclothes officers on the lookout for the ASU, in what was known as Operation Combo. The Met's 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 attacked. They had thrown a bomb through the restaurant window a few weeks before on 12 November 1975, killing John Batey and injuring 15 others. The Balcombe Street siege started after a chase through London, as the Metropolitan Police pursued Doherty, O'Connell, Butler and Duggan through the streets after they had fired gunshots through the window of Scott's restaurant in Mount Street, Mayfair. The flat he was seen fleeing from was discovered to be a bomb factory used by the unit. Quinn had recently shot dead police constable Stephen Tibble in London after fleeing from police officers. The four members of what became known as the ' Balcombe Street gang', Joe O'Connell, Edward Butler, Harry Duggan and Hugh Doherty, were part of a six-man IRA Active Service Unit (ASU) that also included Brendan Dowd and Liam Quinn. In one incident the Guinness Book of Records co-founder and conservative political activist Ross McWhirter was assassinated he had offered a £50,000 reward to anyone willing to inform the security forces of IRA activity. Some 40 bombs exploded in London, killing 35 people and injuring many more. In 19, London was subjected to a 14-month campaign of gun and bomb attacks by the Provisional IRA. Scott's restaurant in 2005, the second attack on which preceded the siege. Carlton Tower and Portman Hotel shootings.

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