Sam McKnight is one of the most famous hairdressers in the world. His clients have included Princess Diana, Kate Moss and Madonna. So when he tells you to wash your hair in bottled water, you wash your hair in bottled water.

In an interview in the current issue of Fantastic Man magazine, McKnight claims that rinsing your hair, post-shampoo, in mineral water is “crucial for washing out the hard water and helps to balance the pH of your scalp”. Hard water and water with a high pH, says McKnight, are both “awful” for your hair. “Crucial” and “awful” being the operative words here.

Speaking to the Guardian, McKnight expands: “Hard water contains higher levels of calcium and magnesium, which can bond to the hair” – making it brittle and dull, he says. “It can affect how hair absorbs and responds to products, so could potentially prevent your treatments from working as best as they could.” In short, a quick rinse in something bottled, he confirms, can really make it sing. Plus, what price beauty?

It’s not atypical advice from an industry fond of grandiose marketing. A new report by consumer analysts Mintel has decreed water “the new luxury” and, jarringly for water companies, predicts that “waterless beauty” – products with low water content or washing without it – will become commonplace. And Christophe Robin, who colours Catherine Deneuve and Tilda Swinton’s hair, recently told the New York Times there is a right way to wash your hair: no more than twice a week.

Hair washing has become increasingly elaborate in the past few years and water is beauty’s latest battleground. A couple of years ago, “double washing” – once to get the dirt out, and once to make it “nice” (as a blogger once told me) – became the norm. But that was small fry. Other micro movements to emerge include the “reverse system”, conditioning your hair before you shampoo – or its eco-friendly sister, the no poo movement, which forgoes washing it altogether.

There are filters that affect what comes out of your showerhead (including vitamin C) and people who change their shampoo weekly for fear their hair might get too “attached” to a product. Whether these work is one thing. But following the notion that your hair might become sentient overnight and form a dependency on shampoo is quite another, at which point, a little mineral water over the sink doesn’t seem so nuts. Naturally, I had to put it to the test. First on the list is a £1.25 bottle of Vittel. I shampoo, condition and rinse my hair with it over the sink. I feel extravagant, then terribly guilty, once I realise there is no difference. The next day I move on to Voss, an expensive Norwegian water heavy on mineral content and which comes in a glass cylinder the weight of a newborn baby. My hair is the same – possibly a little drier, but certainly no better and I feel deep shame for wasting money on nothing. Finally, I try Perrier. My hair feels fine, good even, and “a bit shinier?” according to one colleague. Perhaps the bubbles help, I think. I wonder if I’ve lost the plot.

Now even if you don’t do “treatments”, you get the gist. It’s up there with advice taken from the wife of Emperor Nero who bathed in donkey’s milk, or Michael Jackson and Britney Spears, who reportedly bathed in Evian (although not together); but it’s arguably in the ballpark of Elizabeth Báthory, who is supposed to have bathed in the blood of 650 virgins in a bid to preserve her youth. And with Voss water retailing at a tidy £2.50 a bottle, for those who wash their hair everyday, this is arguably the tonsorial equivalent of “Let them eat cake” – it’s hair-washing for the 1%.

Helen Apps of water service company United Utilities gave her opinion on the matter. Apps isn’t au fait with hairdressing, but she does concede that hardness makes a difference to how you wash your hair. Generally speaking, the north has softer water than the south, although this isn’t uniform – “It varies regionally,” she says. However, from experience she says: “Soft water requires less shampoo and more water to rinse,” but that’s because soft water contains fewer minerals, which are likely to be less testing on your hair. Manchester, where Apps is from, has soft water. It comes from the Lake District, and is mostly rainwater. Water that comes from underground, however, soaks up more minerals, making it dense – or hard. “There’s also more scum in hard water,” she says, which won’t help. Still, this doesn’t explain the benefits of using mineral water: “It strikes me as rather an odd assertion, because mineral water is, by its very nature, hard. That’s why it’s called “mineral water” – it’s full of minerals. As for the cost ...”

Perrier has a pH of around 5.5 – the other waters have high pHs. If Perrier is the most effective, maybe there’s something in McKnight’s theory? Apps tells me, however, that the pH is probably a red herring. It’s hard to tell from a small number of washes, and equally hard to tell given there is no control wash – my hair is dirtier when I do sport, for example. With no conclusion, I decide to forgo McKnight’s advice, abandoning bottled water in favour of flat champagne. This follows the advice of Diana Vreeland, who suggested rinsing “your blond child’s hair in dead champagne to keep it gold, as they do in France” in her Harper’s Bazaar column, Why don’t you?, which featured eccentric lifestyle advice. When wet, my hair feels softer. Encouraged, I dry it before realising it is flat and incredibly sticky. Disheartened, I get back in the shower to shampoo my hair again ... with water.