"It is not a nine-to-five post," a Foreign Office source said yesterday. "The staff need to get in touch with operatives overseas. As a consequence, there are people working at their desks through the night."
The union flag at the front of the building was still flying as the missile found its target. Most government offices lower the flag in the evening, but the SIS has a special dispensation from the Queen to fly theirs all night.
The visible damage was a broken window and charred curtains. Damage to the office was described as minor. No-one was injured, despite the numbers in the building, even at 9.45pm.
The SIS has more than 2,000 staff, some based at Vauxhall and others operating overseas, either attached to embassies as "diplomats" or working under cover.
Those based in the striking block by the Thames yesterday dealt calmly with the attack, according to the Foreign Office. "There was no disruption of operations," a source said.
Even if the damage had been serious, the Foreign Office stressed that the work of the SIS would have been unlikely to have been interrupted. Emergency procedures have been prepared to keep the department running. In spite of all the peoples, groups and countries that the SIS has antagonised down the years, Wednesday night's attack was the first on its head quarters since its founding by the Foreign Office in 1909.
The attack will be seen as vindication by critics who pointed out that the flamboyant new SIS building - designed by Terry Farrell - was too visible a target. Until it moved, it had occupied buildings indistinguishable from other grey blocks round Westminster and Whitehall.
At the time of the move in 1994, the SIS said such a high-profile building chimed with the political mood of the time: that the intelligence services should accept a degree of public scrutiny. For anyone left in any doubt about who occupied the building, it featured in the the James Bond film The World Is Not Enough.
There was controversy over the cost of the building - £230m - of which £85m was devoted to internal fittings. The Foreign Office yesterday replied to these critics, saying the minimum damage caused was a tribute to the trouble taken to make the building as resistant as possible to terrorist attack.
The SIS , better known outside government circles as MI6, is responsible for gathering intelligence overseas. MI5 confines its work to Britain and occupies a discreet building on the other side of the river.
Security at Vauxhall is as high as anywhere in the country. There are guards, video cameras and security barriers. Visitors are taken in by armoured car to an underground car park and then by lift to their destination.
The only member of SIS whose identity has been made public is the head, Richard Dearlove, aged 55, who took over last September. As for the others, the Foreign Office said they did not conform to the public image as Oxbridge types but were drawn from all parts of society. "The reason they are not recognisable in the street is because they look like everyone else," the Foreign Office source said.
