For a number of very good reasons, according to Anna, who doesn't really need the salary from her managerial post. Her father made enough from his business interests and some shrewd investments for neither of his children to have to make their own way. Anna's younger sister has taken advantage of this to eschew the nine-to-five routine and write. But Anna, 29, prefers to build a career, helping to run the European distribution network for a food export company. "I was academically quite bright, did well in exams, went on to do a degree and an MBA and it would have seemed ridiculous to waste it," she says. "I wanted an identity outside just being my father's daughter. And I wanted to make my own circle of friends and acquaintances and colleagues."
She agrees she has had an easier path than many. Her father bought her a desirable four-bedroom terraced house when she moved to London to start her first job, and she has an allowance on top of her earnings. "I actually don't tell people I work with that I have all that," she says. "People are very funny about people with more money than them. I don't want people to think I'm a snob or a toff." Working, she says, is good for you. "It gives you something to think about outside yourself. You feel you are contributing to a team effort. It stops you trailing endlessly from health club to lunch to the shops, which is such a vacuous way of life. And it makes you seem normal. Most of my friends don't have independent means and not having to live like everyone else makes you seem a bit alien."
This is a sentiment echoed by Gary Brand, formerly a part-time bouncer and mini-cab driver, and member of a lottery syndicate that scooped pounds 5.2m. He stopped work, and, he says, "slobbed about the house, watching television and eating junk food". When a fellow syndicate member, equally bored with doing nothing, suggested they buy out an ailing local taxi firm and turn it around, he jumped at the chance. He now works an 80-hour week and says he is "enjoying every minute of it". "Things you have earned are so much sweeter than those that just drop into your lap," he now says.
For trustafarians with little jobs in PR or interior design, a job is almost a lifestyle accessory. But some people with almost unimaginable personal fortunes still choose incredibly stressful employment - like Vivien Duffield, deputy chairman of the troubled Royal Opera House and one of the richest heiresses in Britain since the death of her father, the industrialist Sir Charles Clore. Stella McCartney, successful fashion designer and daughter of one of the richest men in the country, is scornful of anyone who suggests her career is based on who she is, not what she can do. "When I think about how I've worked solidly since school, never even taking a year off, it pisses me off that people imply I'm here only on the family name. Maybe the press would give me an easier time if I was a trust fund smackhead."
Ah yes, those. What happens if you have lots of money and time, but nothing much to focus your energies on? The seventh Marquess of Bristol, John Hervey, 44, died earlier this year after a lifelong history of substance abuse. He was 16 when he inherited pounds 1m, plus a further pounds 4m at the age of 18, a fortune which fuelled his self-destructive traits. Shortly afterwards, the Duke and Duchess of Northumberland went to court to defer their elder son, Earl Percy, 14, from receiving his inheritance until the age of 25. The Duchess's QC described the young Earl as "a clever boy, hardworking and ambitious to cope with the real world and make his own way in it". The Marquess of Blandford, formerly a toff tearaway himself, but now a reformed character, approved. "I do understand, far better than most, what it means to have too much too young," he noted.
It Girl Tara Palmer-Tomkinson has been recovering from strain and exhaustion in an exclusive clinic recently - it seems as though too many parties could actually be as stressful as getting the No 77 bus to the office and dealing with the boss's bad moods. Meanwhile Tamara Beckwith, another of the same ilk, is determined to find a proper job, though her latest stint with Liberty Radio hit something of a glitch on the morning that she overslept and failed to turn up to do the breakfast show.
Pat Thorne of WealthWatch, a magazine produced by Sunrise Publishers, says that attitudes to working when wealthy vary greatly. "Quite often children whose parents have had to work to make their money are brought up with a respect for money and look to make their own names. They feel it's a challenge to do as well as their parents did, often in quite a different sphere - being in parents' shadows can ruin lives."
The money, she says, isn't always the point of working anyway. "It can be the power, the influence, or the desire to do something meaningful. I was reading about a businessman the other day who said he could have retired 20 years ago, but he got his identity from his work. It wasn't about the money any more - that wasn't the driving force." Administering a large fortune can be a full-time occupation in itself, she points out.
Of course, she adds, exactly how much is enough to stop working if you want to depends on your aspirations. "If your idea of not having money worries includes replacing the Roller every year and having staff, then you need an awful lot." To cover the mortgage, have decent holidays and not have to worry about your old age, it has been calculated that around pounds 5m would be the optimum sum.
"But if the idea of having no money worries is covering more modest means," says Pat Thorne, "most of us would be glad to have a mere million quid in the bank."