Jim Rado, co-author of HAIR, recalls the sixties so vividly that he could have travelled through time, skipping the intervening 25 years since the original production. His slow drawl and thoughtful demeanor also suggest someone from a nicer decade. Significantly, he refuses to give his age, not wanting to be pigeon-holed: he is forever young.

True, his blue and green baseball cap, worn back-the-front over straggly blond hair, is an unmistakable symbol of the 1990s; and there's no sign of beads and not a sniff of patchoull. But his tie-dyed, pink T-shirt is clearly a memento of those heady days when he and his friend, Gerome Ragni, together with composer Galt McDermott, preserved in a musical time-capsule what was happening around them on the streets of New York.

Ragni died of cancer two years ago, and Rado, recovering from the trauma of his death, is visiting London for the first time since 1968.

Where did that quarter of a century go? "It doesn't seem like twenty five years to me at all, not at all. It's so incredible to have a marker like Hair in your life. I guess a lot of people have a marker when they graduate from college but mine is when Hair happened.

Rado came to Britain to watch over Hair's rehearsals – this Old Vic production is what he refers to as the "first first-class revival" of the ground-breaking show that even had Princess Anne dancing on stage 25 years ago. And he provided through his memories a living historical database for the present cast, many of whom were born after flower-power withered.

"I've been over here from the first day of casting," he says during a break in rehearsals. "I'm helping to guide it along the right path. It's such a multi-layered piece and I know so much about it, things that may not be available to everyone. So I'm here to make sure everything is known – the meanings of reference, the truths of the moments and the feelings of the characters. I'm here as a guide and reference point."

"I don't know if I ever considered myself to be a hippie. I very much identify with them but I don't think I could live a communal lifestyle. I believe in the raising of consciousness. I believe in love, I believe in romantic love – I'm too romantic for my own good – and I also believe in the larger love, the grander love of people for one and other."

MacDermot, meanwhile, has been keeping an ear on his music. Already 38 when the wrote Hair, he was considered extremely ancient then – mutual friends introduced him to Rado and Ragni when they were searching for a composer – and now he is a grandfather. The son of a Canadian diplomat, he always did dress conservatively rather than freaky, so composer and author/lyricist look an odd couple working together in the rehearsal studio.

But MacDermot, whose hair is turning white and could by no means stretch to "get caught in the trees, give a home to the fleas" explains, "In a way we are very similar. Mentally we are similar, but we lead different lives. I never wanted to be a hippie, I just wanted to be a composer. And as a composer you have to spend time working at a desk or piano – their lives took place on the street."

After the worldwide success of numbers such as Aquarius, Good Morning Starshine, Let The Sunshine In, MacDermot went on to compose scores for other musicals and films of greater and lesser degrees of success – a recent one was Robert de Niro's movie, Mistress.

He admits none of his work since has had the international impact of Hair which he now regards as having been the result of "just a lucky moment."

"You get a bunch of people who are working together and working well – and they produce something terrific. Basically, it was a good idea. When it was first done, I don't think people realised how good the script was and there is enough musical force so it stays alive. I've written tons of shows since this, but not one that works so well."

At his apartment in Hoboken, New Jersey, Rado likewise works on new ideas – stories and songs about love, the environmental crisis, gay liberation – and he holds occasional workshops. Yet he remains deeply rooted and entangled in Hair. It is his baby.

There have previously been small-scale revivals in other countries, and Rado himself directed a simple "bus and truck" version which is currently being toured in Europe by an American cast. But this is the big one, marking the silver jubilee of the flower children.

"I want this first first-class revival to be right-on," says Rado. "I want it to be the next true development of Hair." He also puts a message across about Hair's message – love, peace, harmony with nature – but explaining psychedlic hippie philosophy to Thatcher's children and a society beleaguered by Aids and drug addiction, has occasionally been a challenge.

Indeed, Rado has toned down the drugs element of some lyrics, not so much to bring the show into line with current unpermissive thinking, but because, he admits, he and Ragni got it wrong. They were never really hippies as such. They were young unemployed actors who decided to write themselves a show – Rado starred as Claude on Broadway, Ragni played Berger.

So they weren't personally au fait with drugs when they wrote the notorious chant from "Hashish" – "Hashish, cocaine, heroine, opium…" Now that he has done more research and experimented with soft drugs himself – he hadn't in 1968 – Rado has changed the words to, "Hasish, Marijuana, Magic Mushroom, LSD…" Similarly, the song "Walking in Space" no longer mentions tripping from potsville to 'mainline' but to 'starlight'. Whatever the rights and wrongs of any drug taking, that has to be an improvement.

Rado explains, "The script has been vastly revised but it still remains faithful to 1968. We've reworked some of the structure, examined the dialogue, added new lyrics.

"References to hard drugs have been eliminated and we've included more of the drugs that were particular to the hippie culture – organic, direct from nature types of drugs. They were really part of the awareness-expansion scene that was going on at the time. Hard drugs were not what hippie people were about. Rather than mind expanding they were killing drugs, highly addictive and demoralising. They divided people rather than bringing them together.

"We were mistaken. We didn't know as much about everything as we should have – we were simply doing a listing."

There are a couple of new songs written by the three collaborators before Ragni's death. "They're a little different, but it's still rock'n'roll," says McDermott. He has also scored a new War instrumental and made various adaptions for this cast's vocal range and styles. He may be groomed, but he's a groover. Having taken a degree in music at the University of Capetown, he spent 18 months in England learning to play rock'n'roll – that, of course, was almost before time began.

Rado extemporises in fascinating depth about Hair's meaning. MacDermott comments, "A lot of people don't go to the theatre for a message, they want to be moved."

But however its audience feels (probably a mix of entertained and mentally stimulated) why is Hair being revived now?

"The time is right," declares Rado. "It's a cycle. There's a subconscious human yearning to know about what happened in those days – and also to know what's going to happen. Hair gives things to think about in determining our own future, the way we want to be and the way we can be. I understand the drawbacks and the failures of the hippie movement. But it was an evolutionary time which points the way to something important.

"We must always learn by mistakes and not dismiss an entire movement because there were some negative things developing,

"I'm not a guru, I'm not an oracle. I don't have the answers but I think the hippie thing at its core has a real planet philosophy – and I think that's where we should be headed. Why should we head towards more wars?"

Make love, not war. That's what they said as they dropped out, turned on, stripped off… Actually, unabashed as theatre audiences tend to be nowadays what with even a diva having bared her all at the Royal Opera House, nudity can still make news, especially in tabloid newspapers.

"The hippies represented a breakthrough in the way people related to each other physically," says Rado. "Even when you first met a complete stranger, there would be an embrace and a hippie handshake…"

He takes my hand and demonstrates how to put the hands' Mounds of Venus together, representing love and friendship. Hippies believed the conventional handshake caused a clash of the aggressive Mounds of Mars. After the hippie handshake, he adds, there would have been "the sharing of joy and some mystical revelations."

We didn't progress to such things. Instead, I asked Rado if there was anything else he wanted to say to a London audience. Yes, there was: "We miss Gerome Ragni because he was a free spirit and a very giving and creative soul – and a very funny guy."