The SBS - seaborne commandos - are less well-known than the Army's SAS but operate in even more hostile conditions, as canoeists and frogmen on enemy coasts, delivered to their targets by ships and submarines.
It was the SBS, not the SAS, who carried out the first special-forces operation in the Gulf War, cutting the fibre-optic cable linking Saddam Hussein's headquarters with his Scud-missile launch sites, just 40 miles from Baghdad. Paddy Ashdown, the Liberal Democrat leader, is a former member of the SBS.
The British force at present is modest - about 15 specialists, providing reconnaissance and expertise in the use of specialised equipment.
An estimated 470 Britons are among Western foreigners still in Zaire. Diplomatic sources at the weekend said their advice to Western nationals was still at "phase one" level - do not go to Zaire unless you have to and leave unless it is imperative to stay. There are two more stages; "get out" - by commercial means - and, finally, assisted evacuation, when no commercial means are available.
The main escape route from the capital, Kinshasa, is north across the Zaire river into Brazzaville. On the other side, about 1,200 French and 500 Belgian troops are waiting to help Western nationals to leave the country.
The French and the Belgians both have historical links with the country and their presence is also a reflection of a desire to maintain political influence, rather than a reflection of the number of their own nationals. The United States has several hundred troops in the area, but would play its biggest part by providing air transport.
The anti-government rebels, who control one-fifth of Zaire, say their forces are 100 miles from the southern mining capital of Lubumbashi and half that distance from the diamond-mining capital of Mbuji-Mayi. Johannesburg- based investment analyst John Klemmow told the news agency Reuters that the rebels were already offering gold prospecting concessions, in parts of the country they control, to foreign companies.
Laurent Kabila, the rebel leader, yesterday dispatched a senior aide to represent him in Togo at tomorrow's special Organisation of African Unity (OAU) summit on Zaire's civil war, officials said.
US and French envoys lobbied some 20 African countries to convince them to push for a ceasefire and talks at the summit, French officials said. But regional analysts doubted the success of the summit in the absence of Presidents Yoweri Museveni of Uganda and Pasteur Bizimungu of Rwanda, as well as Mr Kabila.
President Mobutu Sese Seko, back in Zaire after cancer treatment, has not said whether he will attend but his journey home on Friday left him so drained that he sent away an official welcoming committee and did not appear in public until Sunday.
The main scapegoat for the government's failure to push back the rebels handed in his resignation to Mr Mobutu yesterday. The transitional parliament voted to oust Prime Minister Kengo wa Dondo and his government last week, but Mr Kengo at first rejected the move as unconstitutional. The main opposition leader, Etienne Tshisekedi, is the most likely candidate to succeed Mr Kengo.