The latest royal phone-tapping allegations have brought back memories of Princess Diana and that 'Squidgygate' tape. But how easy is it to snoop on someone's phone calls? Lindsay Jennings reports.

CYRIL REENAN twisted the dial on his £900 radio scanner, settled back and began listening. He could hear giggles, the flirtatious giggles of a young woman.

"I shall tell people I'm going (to London) for acupuncture," she said, her tone turning mock serious.

"Squidge, cover them footsteps," laughed her caller.

The conversation, recorded in 1989, lasted exactly 23 minutes. Gradually, the implications of what Reenan had captured on his scanner - which was linked to a 20-foot aerial in his backyard - began to dawn on him.

The most he'd hoped for was to pick up a few racing tips from locals in the horse racing world. But a year later, the former bank manager sold the tape of the conversation, between Princess Diana and her friend James Gilbey, to a national newspaper. Aside from her "Squidgy" nickname, the contents of the tape were sensational, describing Diana's life with Prince Charles as "real torture," confirming that she "binged" and how she sometimes caught the Queen Mother watching her "with a strange look in her eyes".

It also proved just how fruitful even the briefest forays into eavesdropping could be.

Four years after they were recorded by a radio enthusiast on December 18, 1989, details were also published of the intimate late night phone conversation between Prince Charles and the then Camilla Parker Bowles. Once again, the recording was dynamite, revealing the heir to the throne's toilet humour, his desperation to be with Camilla, and an affinity with feminine sanitary products.

Those brief recordings provided a rare and human insight into the House of Windsor at a time when the royal family was barely off the news agenda. Our fascination with the royals and celebrities and voracious appetite for even the tiniest details about how they live has not dimmed over the years. The only changes, perhaps, have been the way in which technology has changed to capture that information.

Yesterday, three men, including the News of the World's Royal Editor, Clive Goodman, were at the centre of allegations that they had attempted to listen to the mobile phone voicemail messages of the royal family, and possibly celebrities and politicians. An inquiry began seven months ago following complaints by Clarence House, the official residence of Prince Charles, after details of a meeting between ITN reporter Tom Bradby and Prince William emerged in the News of the World. The latest security breach is centred on whether voicemail messages on mobiles belonging to three staff members of Clarence House, were accessed and listened to by third parties.

But this kind of accessing is a long way from amateur radio buffs picking up random conversations in their back gardens. Since analogue phone systems were switched off in the UK, radio systems have been unable to tune into them using a scanner. The digital phone systems mean that phone conversations are scrambled and encoded between the phone and the nearest network point. According to security experts, it is possible to tune into them while a conversation is taking place, but it's a difficult process, with the security forces the only organisations which carry it out.

It is far cheaper and easier, however, to access someone's voicemail messages, says Paul Wilson, senior security consultant with Waterstons, a professional services consultancy based in Durham.

It is a weakness in the system whereby a mobile phone that is switched off can have its voicemail accessed by a third party. A snooper can contact a generic voicemail number given by a mobile phone company, and key in the individual number of the mobile and a pin number. If mobile owners have kept the same pin code as the factory setting, it's easier for hackers to access the messages if they already have their mobile number.

"There is the technology to fire off what's known as a brute force attack to unscramble the four digit pin, but my guess is that it's just been left on default," says Mr Wilson. "The person who's telephone it was probably didn't even know the service existed and never changed the pin number."

But aside from hacking into voicemail messages, there are a whole number of ways in which you can pretend to be James Bond. And you don't need Q to point you in the right direction when it comes to hi-tech equipment. Although it is illegal to gain access to another person's telephone under the Regulation of Investigatory Powers Act 2000, apparently it doesn't stop you selling the goods. A quick look on the Internet reveals a number of phone-tapping devices available from as little as £30.

On eBay, a 'telephone recording kit' (known as a bug spy record phone) which works by intercepting the wire from the phone to the telephone exchange, is for sale from a starting bid of £4.38.

More sophisticated bugs convert the electrical current of the phone into radiowaves and transmit the information to a receiver, usually somewhere nearby.

IMSI catchers, used by the real James Bonds, are devices which intercept mobile phones by confusing the phone into thinking it is a preferred base station in terms of signal strength. It logs the numbers of all the mobile phones in the area as they attempt to attach to the base station before determining the phone numbers of each individual phone.

And equally, there is the counter-surveillance available, the so-called bug detectors and anti-phone tapping devices which connect to the telephone line.

Says Dr Lindsay Marshall, senior lecturer in the school of computer sciences at Newcastle University: "The whole point about anything wireless is that you can listen in, which goes for mobile telephones as well. But in the case of accessing voicemail, it seems the weakness is the security aspect regarding pin numbers."

With more than 200,000 cameras and bugging devices sold in Britain every year, it seems to be a buoyant market. But what can we do to keep safe?

Change our pin numbers to access our voicemail messages and keep our mobile numbers close to our chests, according to Paul Wilson.

"I also think the mobile companies should ensure that the voicemail service itself has a mandatory pin from the off," he says. "Accessing voicemails shows that all security isn't about high technology. If people aren't thinking about security from the ground up, effectively it's like locking your door and leaving your windows open."

And to those of you who feel the need to snoop, perhaps you should think of your conscience before you go listening into private conversations. Cyril Reenan died in September 2004, aged 82, after declaring that he "bitterly regretted" making the Squidgyate tape.

"It was the biggest mistake of my life," he said. "To all those who have felt upset and disturbed by my stupid actions, may I say I am sorry."