It was the dawning of the age of Aquarius, or at least the age of theatrical freedom. On September 26, 1968, the Lord Chamberlain's stranglehold on what could be performed onstage ended. It was just in time for the hippy musical Hair, which opened the following night in a fever of anticipation.

Hair came hot from Broadway, where the cast had stripped off at the end of the first act. Before the opening I met its three stars - Paul Nicholas, Oliver Tobias and Annabel Leventon. We were all of an age - 20, 22, 24 - and they were all beautiful, and about to be famous.

The shaggy, smouldering Tobias had studied drama and dance and played in a rock band. Nicholas had played piano for Screaming Lord Sutch. Both boys were sweetly diffident. Leventon, with long blonde fringe and a Marianne Faithfull look, was not. She was the spokesperson, the Oxford graduate, confident, spirited and challenging. “We all go along with the statement behind the show,” she declared. “It is about youth against social conventions. It's very moving and important.”

When I asked the obvious question - “And will you peel off?” - she snapped: “If you ask me that I won't say anything to you at all.” It was several decades before I dared to speak to her again.

Times Archive, 1968: Hair at the Shaftesbury Theatre

Nothing else remotely like it has yet struck the West End

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Forty years on, I sit in her sunny Primrose Hill flat. She is just back from playing Madame Arcati in Coward's Blithe Spirit in France, worlds away from Hair. She puts on the original album and we sing along with the adorable Frank Mills, which we agree always makes us cry. Let the Sunshine In makes her cry too.

We reflect on how Hair willed our generation to be gripped by heady anti-establishment notions of protest songs, pot-smoking, peace-and-love pagan rituals: its refrain of “harmony and understanding” summed up the authentic voice of youth. How outrageous it seemed that the stuffy old Lord Chamberlain should ever have been able to ban or censor plays. How risible that he altered the script of Beyond the Fringe, from “Enter three outrageous old queens. First queen: ‘Hullo, darlings!'” to “Enter three men of aesthetic appearance. First man: ‘Hullo, men!'”

In 1968 Leventon had been in New York so she saw the original production. She decided instantly that she must be part of the London transfer and queued with the drama graduates and unknowns (the only prerequisite seems to have been a head of hair) to audition for the tribe of hirsute hippies. The part she wanted had already gone to the long, lean Rohan McCulloch, who danced like a snake. There was only one white girl's role left: Sheila, the student protester who sings the lovely ballad Easy to be Hard about caring for strangers, and about social injustice. (“How can people have no feelings?/ How can they ignore their friends?/ Easy to be proud/ Easy to say no.”)

After several recalls Leventon was told: ‘The director is coming in next Thursday, can you lose a stone by then?'” She did, in a week. A Harley Street doctor gave her speed and put her on a protein-only diet. Another stone fell off during rehearsals. When the strip scene came up, the director Tom O'Horgan said it was everyone's personal choice: “You don't have to do it.” She'd decided she would not strip - until the dress rehearsal, when she saw uniformed “policemen” marching menacingly down the aisles to stop the show. Catching the onstage mood of defiance to authority, she stripped. The scene lasted a mere 20 seconds and was so dimly lit that the dirty mac brigade was disappointed, but the impact was undeniable.

The critics were duly charmed by the show's music, its light and colour and energy. “Anyone with any life in them will love it,” one wrote. “When Annabel Leventon sings, she just sings. Sings her heart out and reduces herself and me to tears.”

The exuberance was infectious. Everyone in the stalls went onstage afterwards and danced, from Princess Anne to Zsa Zsa Gabor. After the London run, Leventon went to Paris and did it all again in French. Oliver Tobias took the show to Amsterdam and Israel.

All three stars look back with gratitude on Hair, which kick-started their careers (“It was so genuinely good, and had humour and depth, unlike so much of the stuff that came after,” Tobias says), and all have been acting, producing and directing ever since. “It was not only fun but it was a really good musical too,” Nicholas says. “My only regret is that because I was the one singing Where Do I Go? when they all stripped off, I never got to take my clothes off. Missed my chance and it certainly won't happen now.”

All three met last week at another anniversary - the 70th birthday of the Ace Café, the rockers' mecca on the North Circular Road, where Tobias rode his old classic Triumph. They had hoped to put on a charity concert performance of Hair, but the writer Jim Rado wouldn't release the rights - “Hardly in the spirit of the show,” as Nicholas says. Instead they are having a private reunion next Saturday at a Hampstead restaurant. The invitation says: “There will be valet parking for the Zimmer frames.”

“A not to be forgotten experience”: Read our review of Hair at timesonline.co.uk/archive

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