Three years ago, shortly before the first big screen version of their successful TV comedy The Inbetweeners was released, Simon Bird, Blake Harrison, Joe Thomas and James Buckley insisted that the movie marked a final goodbye to the socially inept teenagers we'd all come to love since their debut in 2008.

The film, which follows the lads as they leave school and venture on their first parent-free holiday, went on to notch up the most successful opening weekend ever achieved by a comedy in the UK. And now they're promoting the sequel. It's not for the money though.

"If it was about the money, we'd have brought a film out much quicker, when we were kind of riding the old wave," says Harrison, 29, who plays the lanky, gormless Neil.

Thomas, 30, who fans will recognise as the slightly neurotic and often lovelorn Simon, agrees: "We'd be on about [film] four by now."

"We had no idea it was going to be as successful as it was," adds Bird, who plays pompous, former-briefcase-toting Will. "I think personally, for ourselves and for our careers, we wanted to go on and do other things and not get tied down by The Inbetweeners."

And that's what they did, for the next two years. "It was only that people didn't stop asking us whether we were going to make another film," continues Bird, who turns 30 this month.

"Yeah, there was still this demand," adds Buckley, 26, also known as pathological liar Jay. "Every day, people were coming up to me and tweeting me, 'Please do another film'. If it wasn't for the fans, this film wouldn't have been made."

Although Bird and Thomas, who became friends art Cambridge University, have since collaborated on the First World War-set comedy Chickens, they've all missed working as a foursome. "But that's not a good enough reason to make people watch [another movie]," says Buckley, who's become a dad of two since the first film's release.

"We didn't even want to entertain listening to any more ideas because, I don't know how the others felt, but I felt a bit like The Inbetweeners could be the only thing I do successfully in my life, so let's not ruin that."

However, the show's creators and writers, Iain Morris and Damon Beesley, had been busy working on another film script. "When we saw how funny it was, then everything changed and it became a no-brainer," admits Bird, who married girlfriend Lisa Owens in 2012.

"Yeah, we didn't want to make anything that didn't live up to what the first film has become," adds Harrison.

Any remaining fears they had were allayed during their first read-through in front of the crew; the same team they'd worked with last time. "Afterwards, they all came up to us saying they thought it was funnier than the first film, so we thought we'd made the right choice in agreeing to do it," Harrison explains.

So what fan fans expect from The Inbetweeners 2?

Well, at the time of this interview, much like the first outing, the plot's a closely guarded secret, but they will share certain titbits, like the fact it's set six months after the last movie.

"Jay's gone off to Australia to stay with his uncle and the other three lads are in Britain having quite a rubbish time of it," reveals Harrison. "Neil is working but he really misses his best mate, and the other two lads are at uni but not necessarily having a great time."

As is often the case, their friendship's become fragmented since they left school.

"It's a confusing, horrible period when you've moved away from all your friends and haven't quite made your new set of friends," Bird says.

Then Neil receives an email from Jay saying how brilliant Australia is. "Even though they know he may be slightly exaggerating the truth, they think it's got to be better than what they're doing and head off," explains Harrison - and in great cinematic tradition, there's an element of a road trip.

"It's less romcom-y than the first one," offers Bird. "It's more about the four of them, which is important, I think, because that's what The Inbetweeners has always been about, at its core it's about that group of friends."

No road trip is possible without a car, and it would seem Jay's found something suitably understated for them - a motor with a picture of a topless woman on the bonnet and the words 'mobile virgin conversion unit' emblazoned on the boot.

"Car scenes are the most fun to film, because we're mic'd up so know everyone can hear us, and we can make jokes about the crew but they can't get to us," says Bird.

"We've got a captive audience basically," continues Thomas. "So we babble away with a load of drivel. Basically, our only aim is to make the crew laugh and we're really pleased when we do. It's pathetic, it really is."

This time round, Morris and Beesley decided to step up to the challenge and direct as well, something Thomas likens "to two kids being left in charge of a classroom".

"It was crazy," Bird says, laughing.

"They've always set up this attitude on set that if it's funny just do it, but there's normally a director in place to pull on the reins ever so slightly when things got out of control. So when Iain and Damon took over the reins, it was like the lunatics have taken over the asylum," adds Harrison.

"And," continues Buckley, "any time they did try and discipline us or show us some sort of authority it was completely disrespected and ignored. Well, not only ignored but..."

"Mocked!" offers Bird.

"But despite our remorseless hounding of Iain and Damon, we do absolutely love them," clarifies Thomas. "That's why we did it. That's not an explanation that would wash with them, by the way!"

As hard as it might be for the lads, and their many fans, it would seem this really does mark the end of The Inbetweeners' journey. As Harrison puts it: "The comedy of the show comes from their ignorance and naivety, and if you follow the characters into their late twenties, that kind naivety is almost unforgivable."

It'll be tough to say goodbye though.

"I think it's a nostalgic show in some ways, because people look back to that age as an age of innocence, to a certain extent," says Thomas. "There is something of that which Iain and Damon have managed to capture in these characters, their slight ineptitude but, ultimately, their warmth.

"There's nothing too threatening in The Inbetweeners' universe," he adds. "But at the same time, there's nothing too impressive either."

EXTRA TIME - FROM THE SMALL TO BIG SCREEN

:: Jackass - The wince (and sometimes vomit) inducing antics of the Jackass boys might be Marmite TV, but clearly someone's watching as they've made three movies.

:: Saturday Night Live - A number of characters created for the popular American sketch show have inspired movies, including Wayne's World and The Blues Brothers.

:: Sex And The City - The shoes, relationships and strong women at its core made the series a huge TV hit. A movie followed in 2008 and a less well-received sequel in 2010.

:: The X-Files - David Duchovny and Gillian Anderson found fame in Nineties sci-fi horror series The X Files, which later spawned two films.

:: Teenage Mutant Ninja Turtles - This summer, the Eighties TV cartoon series is given the Hollywood treatment, with Megan Fox starring as April.

:: The Inbetweeners 2 is released in cinemas on Wednesday, August 6