In the world of bugging and interception, sometimes the simplest solution can scupper the efforts of the watchers and listeners.

As the Government disclosed its thoughts on the use of intercept material in trials, the tactics used by targeted suspects reveal the care that trained terrorists, in particular, take to evade detection.

A prudent choice of location such as wide open spaces, woods or crowded places can make it difficult for security authorities to monitor conversations. In the past suspects being followed in Britain are understood to have met in fields where they would lie down before holding whispered conversations to escape surveillance. Their position and their muttered dialogue would have caused a challenge for even the most sophisticated directional microphone system. The problem with this technology, according to Michael Marks of Spymaster, a company that sells surveillance equipment, is that it has a limited success rate. “The microphones pick up all the surrounding noises and it can be difficult to isolate what you want to hear,” he said.

Targeted IRA suspects resorted to meeting in the back seats of buses, according to legend, because of the fear that their private vehicles might be bugged. Their concern appears to have been justified when a listening device was found in 1999 above the lining in the roof of a Ford Mondeo used by the Sinn Fein leaders, Gerry Adams and Martin McGuinness. Officials claimed that the bug was in the car to monitor the driver who was suspected of being an IRA intelligence officer.

Noise is another useful antidote to high-tech listening systems. The traditional method for safegarding whispered conversations is to run the bath or shower or play Beethoven’s 7th Symphony at full volume on the stereo. And as the IRA found, meeting and talking in crowded, noisy pubs also often did the trick.

The belief that sending texts or e-mails on a Blackberry is safe from prying eyes is false. There is no escape if you are being specifically targeted. Mobile phone calls are also relatively easy to monitor, although encryption systems can now be bought on the internet which provide an extra barrier for the authorities to overcome.

Surprisingly, the golden rule for Cold War spies of never storing incriminating evidence in computers or in filing cabinets seems to have been forgotten. Some of the most insightful material, showing the inner workings and networks of al-Qaeda, has been found on coded computer disks.

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