Psst, want to know a secret?

PERHAPS not even the Truth Phone would have helped the Serious Fraud Office in its attempts to prosecute (alleged) fraudsters. A combined telephone, "Covert Electronic Lie Detector", and micro-cassette recorder, the Truth Phone is described as "the world's first telephone that can discern between truth and deception", and is "perfect for all your important conversations".

By analysing voice stress levels with a digital read-out, the machine can differentiate low-stress, truthful responses from high stress ones, which indicate possible deception.

The possibilities for its use are endless, provided the correct 'control' or check questions are initially asked to 'set' the Truth Phone. Innocuous questions can be used: "Is today Tuesday?"; "are you working late at the office?"; "have you ever lied to your spouse?"; "have you been pilfering the Tippex?". The phone's supplier, the Counter Spy Shop, claims "it is 97 per cent effective - the other three per cent are compulsive liars".

Such devices are sold on the basis that information is a high-value currency, whether it is an idea, a process or other intellectual property. And information is more valuable than hard cash because it is portable.

The equipment available to gather and/or protect such information is therefore geared towards the unobtrusive. Pens double as transmitters or 'bug' detectors and can sometimes even be used for writing; filofaxes, staplers, umbrellas, wall-plug sockets and coffee percolators can conceal microphones; and a recorder the size of a credit card can be hidden in clothing. There is even a cigarette packet which records conversations for up to six hours over a distance of six metres.

Cameras can be hidden in an even more ingenious range of items - anything common in the boardroom from a Glenfiddich Malt whisky carton through a cellular phone to a box file. And for that really private telephone call or fax message, there are the straight- forward and secure voice and fax encryptors or scramblers.

For legal reasons, as the suppliers of such equipment emphasise, many of the devices available on the market in the world of security and surveillance are for export only. For the more esoteric (and rich) these include - targeted at the Middle East - a camel racing kit to keep the owner in touch with the jockey, and a falcon tracking system in case our feathered friends get lost. Retailer Spycatcher also stocks an interesting line in literature for the would-be Pierce Brosnan: The Poor Man's James Bond, and Dead Clients Don't Pay - the Bodyguard's Manual.

As to the legality of using such surveillance techniques, there is a general caveat by suppliers that "legal compliance is solely the responsibility of the purchaser". The problem is that such compliance is a grey area. For example, intercepting messages on a public telecomms system without authority is a criminal offence under the Communications Act 1985 and tapping private telephone systems could be considered an offence under the Wireless Telegraphy Act 1949.

It is arguable that only a device which does not infringe the provisions of either Act is likely to be legal as there is no criminal sanction solely for the invasion of privacy - yet.

Similarly, the removal of rubbish from someone's dustbin has led to discussion recently. With some clever legal footwork, lawyers have argued that provided the rubbish is returned, the owners are not being permanently deprived of it, thus getting round the provisions of the Theft Act 1968 should they be accused of handling or receiving stolen goods.

As for whether any private eye instructed by a law firm would use such methods, you might think that, but we couldn't possibly comment.