It was a move by the IRA which broke its own precedent. When the terrorists gave a coded warning before last week's Wilmslow bombings it was supplied to an elderly woman seemingly picked out at random.

Until now, the IRA tended to issue their warnings directly to the police or media organisations. Mervyn Jones, the Chief Constable of Cheshire, whose force dealt with the Wilmslow incident, said he was "amazed" at this new tactic. The anti terrorist branch at Scotland Yard were left scratching their heads as to why the Provos did what they did. And everyone was relieved that the lady had the sense to take the warning seriously.

The use of codewords is essential for the police if they are to save lives before a bomb explodes. But the police need to be sure the codes are being used by "genuine" terrorists, rather than cranks and time-wasters.

Download the new Independent Premium app Sharing the full story, not just the headlines

The procedure has been long established. The codes change periodically by agreement between the Provisionals and the security forces: both sides realise that a failure of communications would be catastrophic.

The chosen name is given by a member of the IRA to the Gardai in Dublin. They in turn pass it on to the RUC Special Branch in Belfast, and from there it is disseminated to forces on the mainland. Liaison is also held with a number of news organisations.

Things are much more complicated in Northern Ireland. There the IRA are not the only bombers. Because of the sheer number of factions, anything up to half dozen codenames could be in use at a given time.

None of the paramilitaries who want to establish a bomb code are turned away. This can sometimes lead to almost surreal problems. One detective recalled: "A few years back a fairly obscure Protestant group phoned up with a bomb warning codename. They kept updating it and each one got more sectarian by turn. Finally they came up with "F--- the Pope", until the officer dealing with them pointed out that, with Catholic officers in the service, "a codename like that was more likely to lead to an explosion than prevent one".

Terrorism expert Professor Paul Wilkinson notes that while the system is not perfect, it is an essential safeguard. "It is a bit like the time the terrorists give to police to clear an area following the bomb warning,"he says. "Sometimes it's nothing like enough - but one has to be grateful that there is any warning at all."