"Can I have a vowel please, Carol?"

Consonant craving: action at the Coblivi:Lon tournament (Steve Anderson)

This simple line will resonate with generations of television viewers. Alongside an iconic(ish) ticking clock and Alan Hawkshaw's famous theme tune it represents a British entertainment institution. To some, Countdown – the longest-running show on Channel 4 – is just a tea-time game show. But to an ever-growing many, it is much, much more than that.

While ambition leads many a plucky armchair player to showcase their skills on the show to Carol (and more recently, Rachel), just beneath the clean-cut surface of the famous blue-and-white studio there lurks a dark and gritty underbelly.

I'm welcomed into this murky underworld outside an anonymous Fleet Street pub at closing time on a balmy August evening by a 23-year-old geologist, Josh Hurst, to attend what he assures me will be a "very special event". This is Coblivi:Lon 2012 and what awaits inside is an all-night, unofficial Countdown tournament, featuring the country's top players. Such events have run since 2005, but this is the first to go overnight and is attracting 33 players – more than any before – including those from as far afield as Perth, Australia, where the Leicestershire-born Hurst now works.

On the day that Mo Farah grabbed gold in the 10,000m final, Hurst tells me with what I can presume is a straight face: "This is the real Olympics. These guys are just as elite as any of those athletes."

The Channel 4 comedy The IT Crowd once featured a storyline in which Richard Ayoade's character, Moss, wins eight Countdown episodes and is thus welcomed into the exclusive "8+" club where octochamps mingle with beautiful women and are revered as rock star-like gods. He's later challenged to a game of "unlicensed street Countdown" by a disgruntled ex-champ. Little did screenwriter Graham Linehan know how close reality can come to fiction.

It might not be quite the seedy, backstreet "8+", but inside series champs and real-life octochamps gather to pit their wits against each other over a seven-hour session. Organiser Amie Bateen reels off a list of players – Graeme Cole, Jonathan Rawlinson, Kirk Bevins – who are all household names in this small world, I'm sure. "There are a lot of big names and a few to watch," Bateen says. "Others are not so good but absolutely love it – Countdown groupies, if you like."

Hurst is one of these ones to watch, having passed an audition for the show, he is awaiting his "call-up" from producers. His younger brother, Jack, at 18 wowed audiences by winning the real Countdown's Series 63 in 2010, smashing the total score record in the process.

Tonight's competitors will have prepared by playing the unofficial online game Apterous.org, set up by Cambridge Ph.D student Charlie Reams as he prepared to appear on Series 59 in 2008. With its 50 variations of the classic game and 3,200 registered players, Apterous hosts around 150 games per day and is the hub that draws these Countdowners – or Apterites – together. In person, players address each other by their full online monikers.

One name on everyone's lips as matches are drawn is Innis Carson, a chemistry student at Edinburgh, who won the show in 2009 aged 17. He is Apterous's top-rated player and a recent winner in the online Aptolympics contest. Internet gaming is all well and good, but he says these live events are what make the hours online worthwhile – even if it means a 10-hour coach ride. "Meeting up in a pub and playing Countdown all night might sound strange to a lot of people but to us, it's brilliant," he says. "We're all nerds."

In a leather jacket and retro black shades, Hurst says this "geeky male" image is one the community is trying to shake off and, indeed, there are several young women playing tonight and ages range from 17 to 61. Ryan Taylor, a 21-year-old gangly student from Hull, takes pleasure in telling me the group has just celebrated the birth of the first "Countdown baby" after a couple of Apterites got together at a tournament. He is here with his girlfriend Michelle Nevitt, an attractive receptionist from Glasgow he pursued after spotting her on the show.

As games commence it's obvious how the hours of online training pay off. While I struggle with four or five-letter words, the seasoned veterans pull out sevens and eights from thin air.

The games themselves are not the polished product we see on TV – there's no glamourous Carol Vorderman or Rachel Riley and no witty celebrity in Dictionary Corner. This is the bare-knuckle boxing of Countdown, using board games, handwritten score sheets and simple stop watches. It is gritty, grimy and raw.

Both letters and numbers rounds are approached scientifically and definitions are disregarded; if a word exists, it's enough.

"What does that mean?" I ask when faced with "ORRERIES"*.

"Who cares? It counts," is the reply.

Lighter interludes lace the less-competitive games as players debate the eligibility of profanities, orifices and sex acts (handjob, yes; blow job, no), as the top scorers fly through the preliminaries. Seven hours drag when you're as hopeless as I am and it all gets a bit surreal at 4am when 61-year-old Phyllis treats us to a Countdown-themed rendition of Coolio's "Gangsta's Paradise".

It's 7.30am by the time the final arrives and through bleary eyes the letters are hard to make out. It's the highly fancied Carson facing Edward McCullagh, a software tester from Newry, Northern Ireland. With the likes of "REMEDIED" and "ANTERIOR" being offered up, it's neck and neck at 113 as the two face the infamous conundrum.

EMOTEEVIL.

Carson buzzes in with TELEMOVIE, and is crowned with the coveted title Coblivi:Lon champ 2012.

As Carson folds up his Countdown God director's chair for the long ride back to Scotland, the rest of this peculiar sect spills out onto London's sunny Sunday-morning streets, ready to dissolve back into a society that is none the wiser.