Simon Bird’s parents wish he’d stop autographing breasts and finish his PhD in philosophy. One of his siblings is a doctor, another is a teacher and the third is a medical student. So his parents are probably scratching their heads about their fourth child: the scholarship boy who read English Literature at Cambridge, but who nowadays has the dubious honour of having “briefcase w****r!” hollered at him when out for a stroll.

If you’re unfamiliar with that indelicate insult, Britain’s most successful youth sitcom in decades must have passed you by. Eye-wateringly lewd and crude, E4’s The Inbetweeners chronicled the trials and tribulations of four sixth-formers, and became a firm favourite with teenagers and adults who ought to know better. The 2011 film spin-off earned £13 million on its opening weekend and an estimated £80 million worldwide, and a sequel has just been announced.

The parents of Joe Thomas – the other Cambridge graduate in The Inbetweeners, who went on to feature in the very funny Channel 4 university comedy Fresh Meat – are more accepting of their son’s unusual career path. “I’m actually surprised by how proud they are,” says the 29-year-old. “My dad admires The Inbetweeners because it takes on issues that other shows wouldn’t tackle...”

“Masturbation,” chips in Bird, helpfully. The actor, who followed up The Inbetweeners with the rather gentler family affair Friday Night Dinner on C4, is coy about how much he’s made from the school hit – although it’s been suggested the four stars will each make £2.5 million for the film sequel. So it’s a surprise to find him of the same mindset as his parents: ambivalent to the point of dismissive about his success. “None of my and Joe’s acclaim or fame has come from anything we’ve created ourselves. I guess I feel that I have stuff to prove.”

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“My heroes were always writers,” agrees Thomas. “I wanted to be Ricky Gervais or Stephen Merchant.” That’s why neither of them minds when critics point out that their characters in Fresh Meat and Friday Night Dinner are awfully similar to their Inbetweeners personae.

“I never came into this industry to be an actor,” explains Bird. “I wanted to be a comedian. All the people I love are being themselves – like Larry David.”

Bird and Thomas’s attempt to prove themselves as adept on paper as on screen comes to fruition with the start of Sky1 sitcom Chickens. First seen in 2011 as part of C4’s Comedy Showcase, and set in the fictional village of Rittle-on-Sea during the First World War, it’s about three men who fail to do their military duty. Bird plays a loser (sound familiar?) with flat feet; Thomas is a pacifist and unlucky in love (again). The third is too busy trying to get his end away, and is played by another Cambridge chum, Jonny Sweet, who shares writing honours.

Sweet also auditioned for The Inbetweeners, for the role of gormless Neil. It’s hard to imagine, as in real life he’s very sharp and terrifically, unselfconsciously plummy – “I’m always just the posh p****.” A better fit would have been Jack Whitehall’s character in Fresh Meat, which he also lost out on.

But if Sweet envies his friends’ moment in the limelight, he’s careful not to show it. The three of them have been penning jokes together since they auditioned for Cambridge’s Footlights in their first term at university. “We all sized each other up as competition-cum-potential colleagues,” recalls Sweet. “Nemeses,” remembers Bird, “and then we realised that it would just be easier to be friends.”

In his final year Bird was voted Footlights president, narrowly pipping Thomas to the post (Sweet then pipped him to the post of vice-president, leaving Thomas with the less glamorous job of archivist). Was he given an address book of celebrated alumni who could let him in the back door of auditions, or introduce him to powerful producers?

Apparently not. “It’s sort of the opposite of the Bullingdon Club,” says Bird. “I think because everyone there is naturally quite ironic and anti-establishment they’re against all that kind of stuff.” The real reason Footlights regularly churns out top-class comedians is more prosaic, he suggests. “Footlights does shows every two weeks with totally original material so you have an outlet there to try things out, and they encourage you to write new stuff. You get three years to fail and experiment without a judgmental audience. It’s a good training ground.”

While they may not deserve a place among the legendary Footlights alumni quite yet, the stars of Chickens have clearly come a long way together. Although probably not far enough for Bird’s parents: he’ll be 30 by the time The Inbetweeners sequel reaches the cinemas next summer. Aren’t they a bit old to be playing foul-mouthed, sex-obsessed teenagers? “We are,” he agrees – squirming a little in his seat – “but that hasn’t stopped us before.”

Chickens is on tonight at 9:30pm on Sky 1

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