In an airy white photo studio near London’s Regent’s Canal, a door opens dramatically, and John Lennon walks out. He arrives in a blaze of neon and tassels, a few daisies perching delicately on one shoulder. He’s then given a French horn, stands in front of a drum between two un-potted plants, and tries to project his trademark acerbic charisma – a particularly hard task when a hairdresser’s taking a fine-toothed comb to your moustache and sideburns.

Under the make-up and the mop top, Paul Merton can’t help but smile.

Fifty years after the Beatles’ Sgt Pepper’s Lonely Hearts Club Band was released, Merton is living the ultimate fan dream – becoming his heroes, John, Paul, George and Ringo, just for one day. He’s happily, you sense almost gratefully, submitted to the all-day RT photoshoot to celebrate the four-part Radio 2 documentary he’s made to mark the anniversary, in which he imagines what would have happened if his favourite band had reunited, made another album, and played live one more time.

“It would have been 1974,” he says, firmly.

He’s worked on it painstakingly, plotting when these events could have actually happened as the Beatles’ solo careers were soaring and diverging, their personal lives rocking and rolling, and their finances still affected by the messy legal wrangles that plagued their final years.

He’s even done the set list for the “concert” and ordered the tracks for the “record” himself, stitching together lesser-known songs from the solo Beatles’ catalogues and live shows.

“To be honest, I’ve been doing this for years,” he says, lowering his satin-clad derriere into a seat between shots, his grey hair stuck down with gel and hair slides (this helps the wigs fit).

“John Lennon said in the 70s that if you want to create a Beatles album, just take tracks off individual albums and put them together, and I always wondered, could you create something that feels right? Then work out when they could have played together, around solo records, having children, trying to get green cards? And then I developed a theory…”

More like this

Merton is fizzing with geeky excitement. “I shouldn’t say any more! But it’s not the four of them in a Transit van on a motorway arguing, you know what I mean?”

Another reason Merton has enjoyed creating an alternative history is a personal one: he became a Beatles obsessive after his heroes split up. Born in 1957 in south London, he was five when they released their first album, Please Please Me, and 12 when they broke up, but he spent his childhood, amazingly, largely unaware of their music.

“I was the least rock ‘n’ roll child ever. My parents listened to the Light Programme, and that was it, really.” He laughs that famous, deep, hearty laugh – it ricochets around the room. “I was completely out of touch with what was happening in pop culture, projecting old 8mm silent films to myself and the wall. My only album was an Al Jolson album!”

But as Merton’s teens whirred onwards, the Beatles’ music seeped in. He’d heard and loved Here Comes the Sun from the last album they recorded together, Abbey Road, while wandering around Woolworths one Christmas, and borrowed the vinyl from his local library. “I remember the librarian going: ‘What, you really haven’t heard Rubber Soul or Revolver?’” He mimics a little boy lost. “And I was, ‘Er… no?’”

Soon he was back every other Saturday, borrowing every album in chronological order of release, like a mini-history project; he was agog at the extraordinary transitions the band had made in seven short years.

“I basically did the Beatles in three months, in fortnight-long bursts, through headphones I got for my birthday, completely lost in their world.” He didn’t share the music with his friends, as he didn’t really go out to parties, he says; instead, he’d stay at home with his clunky tape recorder making compilations of tracks by the band together, and, prophetically enough, apart.

From day one, Merton loved Sgt Pepper. “It’s not a rock ‘n’ roll record, really, is it? It does whimsy very well. It’s also got this strangeness, this hugeness, this going-upstairs-to-have-a-smoke-and-go-into-a-dream stuff. It’s a record that’s soaring above nature.”

As a teenager, he even fantasised he might pay someone to put a photograph of him onto the Sgt Pepper cover one day. “So this” – he gestures around – “is very odd indeed!”

By lunchtime, Merton has transformed into Ringo. He’s in blazing pink, a cap perched on his wig, and cradling a trumpet in precisely the way the drummer holds it on Peter Blake’s famous cover. Merton has a theory about Ringo. “He made the band work, because he came in very late, and they were massive soon after. So he was gung-ho from the off, always making George, Paul and John laugh, who’d already been together years, and having problems with their dynamic.”

Merton’s top five Beatles tracks

Twist and Shout

“The first track where I got the head-shaking. The last track of the day of the Please Please Me sessions, Lennon with a heavy cold, losing his voice — so exciting.”

Ealeanor Rigby

“Imagine the biggest band in the world writing a pop song about an elderly spinster, and putting violins on it. Well they did!”

Tomorrow Never Knows

“Something which had sonically never been done before — sounds reversed, fed through a weird speaker, to the words of Timothy Leary. A pointer to what came next.”

A Day in the Life

“My Beatles’ Desert Island Disc. I can’t imagine never hearing it again. The orchestra driving and pounding, John’s voice soaring over it angelically — it’s like Beethoven’s Fifth.”

You Know My Name, Look Up My Number

“A fantastic comedy piece, like pre-Monty Python stuff, very Goons-like, actually. Done at Paul’s place in Sussex. Spike Milligan lived not far away, so they visited him.”

It’s also telling that Merton’s love of the Beatles deepened when he found out they were fans of the Goons, and that their producer George Martin made comedy records before signing the band to EMI.

“And have you seen the Beatles on Morecambe and Wise in 1963?” he asks fondly. “Eric’s there, asking John what it’s like to be famous, and John’s squaring up to him, saying, ‘It’s not like in your day, you know.’ They were so fast and funny, it’s brilliant.”

Lennon is Merton’s favourite Beatle, who once said he’d rather have been a comedian than a pop star, although Merton himself gives this idea short shrift. “Rose-tinted glasses there, I think, John. I mean, when you’re in a band, you haven’t got another band heckling, trying to get you off the stage all the bloody time, have you?”

One direct influence Merton has taken from the Beatles, however, is their commitment to their craft: “I remember Paul McCartney saying they worked hard to make every album different, and that ethic is one I’ve taken up.” He applies this idea to every new series of Have I Got News for You, he says, without irony, a show he’s now been doing for 27 years. “Nobody says I look tired or bored in it – although, yes, I sometimes looked p****d off because of who I’m sitting next to. But I’m genuinely there every week doing the best I can do.”

The show feels different recently, he says, given what’s going on in the world. “In our first programme, we showed Donald Trump, then North Korea, and I could feel the audience tightening up, which was strange. Our job is to untighten them, really – it’s better they don’t tighten up in the first place. So the next week we did houmous disappearing off supermarket shelves, of course.” Another huge, studio-rattling laugh.

Merton’s done Have I Got News for You half his life – he’s 60 this summer. Does the Sgt Pepper track When I’m Sixty-Four, with its worries about ageing, have a personal resonance now? A firm head-shake. “Sixty-four is nothing, is it? I mean it’s easy for me to say… but it’d have been called “When I’m Eighty-Five” today. People don’t slow down now.”

Merton certainly doesn’t. As well as HIGNFY and weekly London Comedy Store gigs, he’s recently embarked on an unfulfilled dream: writing a film screenplay that he’s trying to get made. “It’s a comedy thriller,” he explains, nervously. “And it’s like starting again. I have no reputation or anything in film, so I’ve had rejection letters. But it’s in development now.” He tilts his head, smiling. “We’ll have to wait and see.”

For now though, for today, Merton’s got the rest of his teenage fantasy to follow through. Here he is as George, in burnished orange with a piccolo, and now in bright sky blue as Paul, the only Beatle he’s ever met – in a corridor at London’s LWT studios, 25 years to the day that John Lennon had died.

“I found out the news from a friend during a call in a phone box, and when I came out somebody had scribbled ‘Power to the People’ [a 1971 Lennon single] across it. To think that 25 years later, I would have Paul McCartney embracing me in a corridor… it’s like doing this. Remembering the boy that I was, in my bedroom, making tapes. And now there are people going, ‘Let’s hear what you’ve done for this Beatles album, then!’”

Under the neon and the tassels, a moustache is definitely twitching. “If you think about it too much, it would send you round the twist. It’s like waiting for the world to catch up with you or something!” And with that, one of the Fab Four with which the world is still catching up, Paul McCartney, returns to the camera for his last performance. Ladies and gentlemen, Paul Merton is finally a Beatle at last.

Paul Merton’s Beatles is on Monday 29 May at 10pm on Radio 2