This special relationship is the subject of BBC2 documentary Shampoo, which reveals that 90 per cent of women canvassed named their hairdresser as the person they would be most likely to confide in after their partner. "Unlike husbands and lovers, a woman's hairdresser is always there to listen," says Michael Rasser, co-owner of top London salon Michaeljohn.
The VIP room at Michaeljohn is hidden away behind Rasser's office. It is pure minimal and, most importantly, private. On my visit, the VIP de jour was singer Alison Moyet. "It's not about hiding from 'My public'," says Moyet, "but there are days when you feel crap and don't want to be seen with a head full of tin foil." Sure, you wouldn't expect to see Shirley Bassey reading the stars under a dryer.
For certain clients it is very important not to be seen. "We do have actors and politicians who have their hair dyed and don't want anyone to know," says Rasser. "But you don't pay to use it and there's no policy on who can and can't use it."
Michaeljohn's regulars know about the VIP room but, like Joanna Lumley and Patsy Kensit, choose not to use it. "It's a hive of gossip," says Johanne Ashworth, a customer of 10 years. "It's a bit like a party atmosphere and definitely not like going to the hairdressers. The only time I went in was to use the phone and I ended up sitting in Earl Spencer's lap."
Of course, it is an unwritten rule that neither staff nor clients tell tales out of school. "You can't be too careful," says Rasser. "I have had journalists coming in here undercover and trying to grill me for royal gossip. But you get wise and never kiss and tell."
Nicky Clarke's plush VIP room at his Mayfair salon is a second home to the likes of David Bowie and Gwyneth Paltrow. With its own separate entrance, inside it looks like the royal box at the Palladium. "The relationship between hairdresser and client is always an intimate one," says Clarke. "With high-profile clients, an element of privacy is vital. Also, having a separate entrance to a VIP room allows for added exclusivity and security."
In Rasser's office at Michaeljohn are countless signed portraits of Princess Anne, the Duchess of Kent and the Duchess of Gloucester. "When I first styled a royal," says Rasser, "I was bobbing and bowing like crazy. But you build up a relationship over the years. Princess Anne is one of the wittiest women I have ever met."
In 30 years as a VIP hairdresser, Rasser has only been fazed once. "When I was doing Liz Taylor's hair, she threw the Krupp Diamond at me and told me to polish it. Now that's what I call a VIP."
Modern Times: Shampoo, BBC2, 9pm, Wednesday 30 April.