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When Paul was about 15, he and John Lennon used to take the bus over to the other side of Liverpool to visit a guitarist who knew how to play the B7 chord – essential for rock ’n’ roll.

Pooling their few pennies, they’d excitedly ride over to sit at the feet of this guitar guru and watch fascinated as he formed the chord shape they could add to their repertoire of E, A and D.

More than 30 years later, when Paul was recounting this tale, he added: “We thought that guy was really old for a musician – he was probably only about 26 but we thought he was really old to be still playing.”

Today Paul is 70 and still playing at an age when most people have swapped vim and verve for slippers in front of the telly. And Paul will never stop.

For the 15 years I had the privilege of working for him as his publicity chief, I witnessed his relentless drive and extraordinary energy at first-hand.

* 70 classic photos of Sir Paul McCartney from the Sixties onwards

And he is about to perform the biggest show of his unparalleled life, playing to a TV audience of more than one billion by closing the opening ceremony at the London 2012 Olympics.

It’s Britain’s greatest contemporary moment and, frankly, there is no one in the nation who is bigger or better placed to symbolise to the planet the true glory of achievement of this country.

He is, of course, used to playing to huge crowds. But Paul’s Olympics show is more than another big gig. In those three or four songs, to the eyes of the whole world he will be Britain.

Not David Cameron, not HM The Queen, it will be working-class Macca – ‘some scruff from Speke’, as he dubs himself – who will represent the very best of British. At 70.

After seven decades, Paul is still doing what he’s spent his life enjoying – breaking the rules.

He’s always done it. It’s like when he wrote the beautiful Beatles ballad For No One, scoring the French horn solo with producer George Martin.

As Paul could not write or read music, an inability which he stubbornly refuses to correct to this day, he sang a note which George pointed out did not even exist.

"The horn player won’t be able to play it, Paul,” said George, “that note is beyond the range of the instrument.”

“Stick it in,” said Paul, “let’s see what happens.”

The result, as we know, was something of beauty, the Macca instinct was right.

(Image: Phil Harris/MirrorPix)

Some might think that after the Olympics he will finally pack away the Hofner bass, stop the music that just comes into his head like it did to Beethoven, and retire with Nancy to do those things he erroneously predicted for himself when as a teenager he wrote When I’m 64.

He once said he’d probably die on stage, “wheeled on in a wheelchair playing Yesterday”.

He appeared to be joking but I’d have a fiver on it because it’s impossible for him to stop the songs coming into his head. They just do.

I’ve seen many a disc jockey test him on that. “Go on,” they say, “see if you can make up a song now, right now, in this studio.”

“OK,” says Macca every time, “give us a sec” – and then he does.

It freaked out Dustin Hoffman, that gift of Paul’s. The actor and Macca were dining in the Caribbean once when Dustin challenged him.

“Give us a subject,” said Macca. “Picasso,” said Hoffman.

Paul pursed his lips in thought for a bit and then began singing, “Drink to me, drink to my health, you know I can’t drink any more”.

The song became the Wings hit, Picasso’s Last Words – at least it did once the amazed Dustin had stopped running around the restaurant shouting, “He’s doing it! He’s actually doing it!”

But what drives this extraordinary man, this council estate kid, to keep doing it?

What I saw close up, on a day-to-day basis, was up-and-at-it Macca, passionately applying his work ethic DIN – Do It Now.

Clearly, his 40-plus years of vegetarianism are proof that, at 70, if you want to front the non-stop, two-and-a-half-hour, 36-song set that is his current show, then you’ve got to eat right.

But there’s more to it than that.

If you ask Paul what is the secret of his success, he’ll probably go coy and say that he doesn’t know. But if you’re lucky, he’ll tell you he believes “it’s longevity” that matters.

Not doing it, not having done it, but to keep on going. “The hardest act to follow,” Paul will tell you, “is yourself”.

Given that since The Beatles first began to shake the world 50 years ago with the 1962 hit Love Me Do, through the 70s with Wings and since the 80s as a solo artist he has had more hit records than any musician before him or since, that is a tough act to follow.

(Image: Adam Sorenson/MirrorPix)

Yesterday alone has now been played on the radio more than 10 million times. “Yeah,” Paul says, “it’s like being a boxer alone in a ring, punching yourself.”

He’ll also tell you one of the main reasons he keeps playing is because he hasn’t quite got where he wants to yet.

Back at the end of the 80s, I was struck by hearing him say, “I’ve got this feeling that I want to write one more really good song”.

I was agog. This was the composer of Let It Be, Hey Jude, Maybe I’m Amazed and god knows how many other classics.

This was the man who soundtracked the lives of billions of us, the one A-level likely lad who became the richest, most successful musician who has ever lived.

And he wanted to do more?

“Yeah,” he said, as if it was obvious.

As it happens, he has possibly written that song. Most of us haven’t heard it yet, but he’s been tinkering with it for more than 10 years now.

It’s called Celebration, part of the symphony Standing Stone, knocked off in between topping the album charts, exhibiting his abstract art, making films and publishing poetry.

And there’s another reason he keeps on – because nobody in rock ’n’ roll has done so yet.

As he’ll tell you, rock ’n’ roll only really started around about 1956 so it’s not yet old enough as an art form to have had people of 70 still playing it.

Like Bob Dylan and his other friend Keith Richards, Paul is bringing rock ’n’ roll to a new maturity.

"I fancy getting like the old bluesmen have, like Muddy Waters or BB King,” says Paul, “getting older but still playing. Nobody ever told Muddy Waters he was too old to play.”

CLEARLY, with a reported fortune of £665million, Paul doesn’t do it for the money. As he says, “you don’t work music, you play it.”

Yet despite the incredible drive Paul is just as humanly frail as the rest of us too – he does weep over a soppy film.

He does like a celebratory Scotch and Coke – “four’s my limit, four and I’m anybody’s” – and he does possess a bugger of a temper and the colourful language to go with it.

But as Linda used to say “that’s allowed”. Why should he be the only man on the planet who’s not permitted to on occasion lose his temper?

He’s human too in not ever wanting to read reviews – “because it’s distracting on stage. Somebody once wrote of a gig that Ebony And Ivory was the low point of my show.

"I dropped it from the set a while after that because I kept on thinking, ‘Oh God, now we’ve reached the low point of the show’.”

(Image: MirrorPix)

And he’s human, and distinctive, in that he has never lost hold of his sensible, working-class roots. Roots that make him hold honesty, politeness and decency as absolutes.

As he once said himself: “I’ve met prime ministers, presidents and royals but I’ve never met anyone with more wisdom than the common sense of the ordinary Liverpool working man.”

He is distinctive in his industry too, in that he’s kind with the little gestures.

He never forgets an employee’s birthday, stops his limo at a florist to go in – not to send in his driver – to buy a bunch of flowers for his wife, stops everything he’s doing backstage if he sees someone upset who needs a hug, and paid tens of thousands of pounds for a private jet to fly a sick child of a friend for treatment at Great Ormond Street Hospital.

And children love him, which is a telling sign of a good man.

I remember we were in Paris once, inciting something of a riot by Paul appearing at a FNAC record store for an impromptu album-signing and press conference.

The cops couldn’t cope with the crowd – Maccamania in spades.

And right at the back of the huge, berserking crowd was a young mother with a severely disabled child.

“Hang on a sec,” said Macca, seeing all as usual. He indicated the mother and his team brought her through the crowd along with her little girl.

“Hello,” said Paul, “What’s your name? You can sit on my lap for the rest of this press chat. Right lads, next question?”

That is the mark of this truly great, ordinary man – a big heart.

And that is what still makes him tick, that love thing The Beatles so much sang about.

Which links to something else I once heard a music writer say about him – “through his songs Paul McCartney has probably brought more joy to more people than anyone alive on this planet”.

And that’s also why, at the age of 70, the scruff from Speke has been chosen to best represent Britain at the biggest show in our nation’s history – because around the world ordinary people understand that Britain has never bred anybody better.

Happy birthday, mate – you deserve it, you’ve done us proud.