Whilst most people realise that Grateful Dead T-shirts remain out of bounds, they're often not too sure whether they can safely ditch the dark colours and suits. It can be difficult to know just how much individuality is allowed. Some women have tested the water by replacing jackets with smart cardigans, and skirts with trousers - even daring to wear patterns and pastels. Men have gone for louder ties, and suit fabrics are increasingly light in weight and colour. In summer, the odd few men have even opted for the "shorts suit" (although this extreme reversion to public school uniform is enough to frighten traditionalists straight back to their heavy, pinstriped suits).
Mary Spillane, who runs the image consultancy "Colour Me Beautiful", says summer brings out the worst interpretations of the latest City image. "When the sun comes out, British brains seem to get fried. Women are particularly vulnerable because out come the armpits, bare legs, and toes hanging out of shoes - which break down all professionalism. There's really no excuse with such wide availability of linen and other lightweight suits, and long dresses that look smart with tailored jackets. The more skin you show, the less authority you have.
"The City is still the most conservatively dressed culture in the universe," she adds. "While the lime-green suit may be the hot West End look, those who work in a City business should buy only the jacket, and wear it with a smart navy dress. That way, you still fit the serious image of the finance world."
For firms that haven't moved on, these dilemmas may remain alien - as they do to Sarah Cross, compliance associate for a fund management company whose employees recently gasped when a colleague turned up in a green suit. But all is not lost for her, if only for one day a week - thanks to "dress down Friday".
Dressing down began in California in the mid-Eighties, when it dawned on employers that conservative attire clashed with their New Age innovation. Around 90 per cent of US companies now allow casual dressing for at least one day a week. The trend later moved to Britain, and European take- overs have helped to liberalise men who, until recently, were strolling about in bowler hats and rolled umbrellas.
It's not only the least conservative firms that offer this cost-free perk. Employees of Warburg - one of the most traditional City banks - can dress down every day, and Sarah Cross's conventional firm is one of many that even allow jeans on "dress down Friday". "I like the classic suits from Monday to Thursday, but on Friday I can freely get on with filing and rummaging around in boxes, as well as feeling relaxed in time in anticipation of the weekend," she says.
Judi James - many of whose clients admit they don't work so effectively on "dress down Friday" - claims it is illogical to introduce these days just because employees like them; they'd probably like even more the idea of having Friday off. Research into American companies shows that casually dressed employees take an average of two minutes longer for lunch. And since most companies insist on formal dress when meeting clients, wearing casual clothes can advertise that you don't have a deal going. Diana Mather, author of Image Works for Women, adds, "90 per cent of our opinion about someone is formed immediately. That alters if there is much further contact, but the danger is that there may not be. You never know who you're going to meet - even on a Friday."
Foreign take-overs aside, fashion itself has encouraged a less structured look. Even Dolce & Gabbana have changed direction from sharp tailoring to casual, crumpled suiting for men. Lilly Anderson, of French Connection, says, "Within the last year, more City women have been buying suit jackets to wear with dresses, and complete suits are often dressed down with T- shirts. Many feel happy in trousers and fitted cardigans, too." However, some companies, such as Mison Recruitment, still don't allow women to wear trousers. "It's really difficult for temps, who have to learn a new dress code in each company," says the managing director, Gill Gaine. "So our ethos is total conservatism, to be safe."
Another theory is that a lot of the real money is now in IT, where twentysomething nerds simply don't feel at ease dealing with bankers in pinstriped suits. Judi James disagrees: "The IT industry has always had its badge of defiance in terms of clothing. If bankers copied that, IT workers would have to turn to ripped jeans. It would be like seeing your teacher in T-shirt and jeans. You'd feel you'd have to be even more informal."
Whether casual on Fridays or every day, experts are worried about shoring up power structures. Anne Duke, Marketing Executive at Commercial Union in the City, notices that her manager doesn't participate in "dress down Friday" - which can lead to segregation (next step, executive washrooms). And women are at a particular disadvantage, says Christopher Early, professor of organisational behaviour at the London Business School. Research reveals that when women aren't wearing a jacket, 80 per cent of people surveyed think they are receptionists or secretaries. Yet Tom Foxcroft, a broker who normally works in tracksuits and jeans, never faces such problems. He does, on the other hand, voluntarily wear a suit from time to time to pull himself back into work mode.
With such uncertainty in dress codes, the advice to dress for the job you want rather than the job you've got suddenly seems somewhat inappropriaten
Jeroen De Rooij, 26, accountant
"We have to wear a smart suit and tie. I feel more comfortable wearing a suit to work - more professional."
Dorita Sheriff, 30, training manager at a law firm
"We don't have dress-down days. I find it easier to buy a professional easy-to-wear suit from Marks and Spencer. People are judged on their appearance and the law is very traditional."
Mike Jones, 35, arts film and video distributor
"I don't have to wear suits to work, I think this creates a better and more relaxed office environment. I would dress up for an art gallery opening, but not for the office."
Belinda Teal, 26, legal secretary
"We are not allowed to wear jeans in the office, but I prefer it that way as I like to wear designer clothes for work like this jacket by Karl Lagerfeld. My dress is from Wallis."
Photographer: Andrew Buurman. Interviews by Alison MoloneyReuse content