Time was when a reporter looking for a private address would go into a library and spend hours poring over an electoral roll. Or, to trace a home from a car numberplate they would write to DVLA describing a fictitious bump and request the address for insurance purposes. Did nobody in Swansea wonder why so many potential suitors of young Royals seemed to be involved in non-injury collisions?

As for checking a criminal record, you just had to know an obliging police officer who would make a quick phone call for you.

I was once arrested for “interfering with the Royal Mail” for trying to get suspected terrorists’ names from an IRA safe house by getting a postman to show me the envelopes from his bag. No charges were pressed.

The rise in computer databases created an opportunity for Fleet Street to reduce the tedium and, sometimes, to outsource the skulduggery to private detectives. An information market arose selling details about individuals so that hackers could trace their movements, benefit claims, shopping habits and phone calls.

Newspapers have long paid for stories. The popular press in particular relies on scoops acquired with a chequebook. On the Daily Mail, I found that £7,500 was the magic number that turned protests about an invasion of privacy into a willingness to talk.

But the new phenomenon was the arrival of professional information mongers. Any hacker knowing the right passwords could get access to personal computer information — often at call centres. The private detective just needed to pose as, say, a health worker to check details. Once one newspaper started hiring these gumshoes, rivals were obliged to follow. It became standard practice to ring an investigator and request all manner of information to avoid being beaten to the story.

The amount of detail available could be remarkable. For £50 or £100 it was possible to get an address from a car registration number; an individual’s social security claims from their address; travel arrangements from computerised flight lists. Do you know the most important detail to trace the most reclusive person? Their GP’s address. Everybody needs a GP to access the health service and the family doctor is local, so it is easy to narrow the search. The best key to getting useful information from databases? A date of birth.

In practice, there seemed to be no law against all this. Journalists were far from the only market for these private investigators or PIs — their other main clients were the legal profession. The Data Protection Act forbids the misuse of private data, but there is a public interest defence. Journalists would usually be confident that they could mount such an argument should the need arise. The tide turned with the Regulation of Investigatory Powers Act of 2000, which dealt with phone tapping. By defining the narrow circumstances in which it was legal to intercept telephone calls, everything else was outlawed. Now only the police and spies can tap phones legally.

The Act coincided with the rapid growth of mobile telephones and voicemail. PIs had been able to provide two main types of information from any phone number: “Friends and Family” or recently dialled numbers. But any child could intercept voicemail as a party trick. Mobiles provide remote access to messages protected by a password, but most users forget to set a password so it remains a default, such as 1234, and can be easily guessed.

Sunday newspaper reporters are particularly vulnerable to laws against intrusion because they survive entirely on exclusives and there is a great temptation to push the envelope. It is be hard to offer a public interest defence if a pursued celebrity has been harmlessly pursuing their private life.

Today the line between legitimate and illegal databases is still a bit shaky. Every newspaper has some form of searchable electoral roll to trace home addresses and phone numbers quite legally. Access to the computerised births, marriages and deaths register provides crucial detail about generations of family history. Databases of company directors provide dates of birth and home addresses. These are all paid-for services.

Other databases such as car numberplates, credit history and criminal records are protected by the Data Protection Act — although not if the public interest can be invoked. What may seem like an intrusion into privacy, a search of a database, can lead to information that is clearly in the public interest to disclose.

What about the tip-off that a woman deported to Britain after a welfare scandal abroad is living off social security benefits in this country? Without using a PI to find her address, it would have been impossible to interview her about how much she claimed.

What about a journalist’s investigation into a mosque linked to radical terrorists inciting teenagers to racial hatred and the killing of Jews? Is it wrong for a reporter to find out a suspicious preacher’s home address by checking his car number?

The Information Commissioner has warned journalists that he will prosecute for infringements, and in practice there is a daily dilemma about what is acceptable. When a suspected serial murderer of prostitutes was arrested and his car seized by police, did the Press check the numberplate to get his identity? You bet they did — and the interested public read about it the next day.

dkennedy@thetimes.co.uk

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