When it comes to affairs of the heart, technology cannot advance quickly enough. "There's more at stake today," O'Neil explains. "In a divorce, men are more likely to seek custody of the children these days than say 20 or 30 years ago. Because there's more to lose, it's not uncommon to use surveillance equipment to keep at the cutting edge."
Conrad Sandler, owner of the three-year-old Knightsbridge spy shop Spycatcher, has also witnessed what he calls, "a regular matrimonial clientele". "Although it is only really rich people who can indulge themselves in this way," he adds. "But if it's a pounds 2m-pounds 3m divorce then it's worth paying pounds 3,000 for proof."
Suspicious spouses will normally spend at least this amount for powerful transmitters and microphones concealed in pens, calculators, or even credit cards. Simpler bugging devices cost about pounds 200 and can be fitted under tables or chairs. Recently, a vengeful male customer spent about pounds 30,000 in Sandler's shop to eavesdrop on his wife's affair. "He bought a system you can install in your London flat and then dial in through your PC from anywhere around the world," he explains. "He's a very wealthy man who wants to protect himself from an expensive divorce."
Despite this dog-eat-dog end of the market, the shop's promotional blurb is strangely gleeful: "For the female executive, how about a handbag with a bug in? Better still, find out what your husband's really up to and give him a smart new mobile phone with a bug in for his birthday!"
Sandler admits that once the female has decided to monitor hubby on his cellular, she's anything but light-hearted. "It's not exactly like selling them a Louis Vuitton handbag where they're elated after they've bought it," he says rather regretfully.
The long-term consequences of their purchase can be even more distressing. Lindy Grant, head of the female detective agency S & Elle Investigations, is sceptical about the advantages of personal bugging. "I wouldn't advise it unless you want to end the relationship. I've had so many cases where clients have listened to tapes and been devastated."
Another drawback is the legal aspect. O'Neil has to warn customers that it is an offence to connect anything to a telephone line without British Telecom approval.
Tessa, a physiotherapist in her early forties, was distraught when, two years ago, she suspected that her husband Ian, a doctor, was having an affair. Although they are now separated, her main objective wasn't divorce but an irrational need to prove her gut instinct right. "It got to the point where I knew he was meeting this woman - I even found her cheque card on the car floor. But he kept denying it and saying I was being irrational. That made me more determined to catch him out."
A private detective advised her where to buy a bugging device for her telephone. "I was physically sick after I played the first tape back," she says. "They were so intimate. Above all, I couldn't believe the hurtful things he said about me to appease her." He had to own up to the four month affair which ended shortly afterwards. "Even then, there was no way I could stay with him. Because of what I'd heard, our break-up was a particularly bitter and painful one."
Wendy Beauchamp-Ward, a solicitor working with matrimonial cases, would rarely recommend this approach. "I think a lot of it is about power games - one party trying to put something over on the other. And you can get divorced easily enough without it."
Yet bugging devices can offer the ultimate temptation to the betrayed, a unique glimpse into a world shared exclusively by the partner and the lover. Such temptation carries a high emotional price. As Grant points out: "It's a huge step to take. I've heard so many people say, 'God I wish I hadn't heard that.' " For a fraction of the cost and the heartache, why not invest in a Louis Vuitton handbag instead?