Bending down, she peered into the depths of a display case. After a while she said: "I want a ring ... like a wedding ring, only with diamonds. I forget what they're called ..." Two pads of eternity rings arrived for inspection, only one within reach. Poking at the hoops like someone testing chocolates for soft centres in the cinema, she squinted at the second pad glinting on a shelf behind the assistant.
"Can I see those? Don't know my size."
After much deliberation, Barbara insisted she'd seen the ring she fancied cheaper elsewhere. Finally, she announced she'd "think about it".
Mercifully, my mission with a mystery shopper seemed to be over. But not quite: suddenly resembling the supervisor from hell, Barbara opened the post-mortem.
"A lot of the pads in the windows weren't straight, Did you see the carpet, all the bits on it? What about the way she was dressed? Not exactly a business-like image, was it? And she didn't ask why I wanted the ring. I had my story ready, it was to be a present from my husband for our 30th wedding anniversary. She should have asked. People like to talk about themselves. With an eye on another sale, she might have asked if he was going to have a ring, too."
Otherwise, the assistant hadn't done badly, Barbara said. She'd run through the differences between nine and 18 carat gold, throwing light on the points system used for diamonds. The price guarantee refund, if the ring turned out to be cheaper elsewhere, was mentioned, along with the 16 days' grace offer for a full refund - in case of a change of mind. And with an eye to security, the assistant hadn't brought out more then two ring pads at a time.
All over the country, a growing army of mystery shoppers like Barbara is deployed to test retail techniques. They work in almost every shopping sphere imaginable: while some mystery shoppers will be having eye-tests, others obscurely mimic golf-course personnel in the market for costly bunker-rakers.
"This isn't a job for shopaholics or bored housewives," says Dawn Edwards of the Tern Consultancy, a Shropshire-based firm that recruits mystery shoppers. "It calls for commercial awareness, keen powers of observation, a retentive memory and the right kind of report-writing skills."
For high-street retailers, the advice provided by her agents can help revive flagging sales. Few purchases - groceries included - are bought solely on price. One way of boosting sales, particularly of non-essential goods, is to improve standards of service and the ambience of a shop. Yet service, always considered to be of prime importance in the US, continues to get overlooked in our nation of shopkeepers, part of the problem perhaps being that many of those entering retailing today have grown up accustomed to self-service shopping.
Entrepreneurial and successful retailers want to be put to the test. Robert Wade Smith, managing director of the fashion sportswear and leisure- wear specialists Wade Smith, whose Liverpool business forms part of the city's re-vitalised Cavern Quarter, has no doubts about the value of mystery shopping.
"We weren't as good as we thought in many areas," he says. "When a few mystery shoppers hit our departments, their reports were eye-openers. It's not a case of a witch-hunt to catch people out: we want to encourage and acknowledge the efforts of our staff as much as possible. We have a philosophy of building long-term relationships with customers. Anyone coming into the building is potentially a customer for decades who might spend pounds 50,000."
So demand is growing for mystery shoppers. Jean, 43, who trained as a librarian, will set out on a mystery shopping assignment wearing something more restrained than usual from her predominantly beige wardrobe. ''You don't want to draw attention to yourself for this work", she says.
James, 45, says: "Employers spend small fortunes on staff-training so it makes sense to do a little checking. But I don't go for individuals - implying a mad axeman is running a branch. I'm more likely to comment on the age-old problem of staff avoiding eye contact, hoping someone else will serve you first."
James sports the occasional bright tie but "dresses down" for mystery shopping trips in open-necked shirts and sweaters. "By training, I'm an accountant. It's second nature to be grey," he explains.
Remaining unmemorable is of prime concern for both Jean and James: openness may be OK for MI5 now but if mystery shoppers have their cover blown, a career can be left in tatters.