As Peep Show returns, David Mitchell and Robert Webb talk to Tara Conlan about the cast's new agonies, their own doubts over Sky's commitment to the arts – and why comedy outranks drama

Sitting forward in an earnest fashion and pushing back his flop of brown hair, David Mitchell is explaining why the production company he has just formed with Robert Webb has gone into business with the BBC. At a time when a growing number of comedians, including Steve Coogan, Julia Davis and Chris O'Dowd, have been beating a path to Sky, it's a bold move from the stars of cult hit Peep Show.

"Sky is making a lot of comedy at the moment, and that's great," says Mitchell. "It's always good to have more choice for pitching ideas and more choices for the viewer. But my worry is that that money could be turned off in a second if someone in News Corp or whatever goes, 'Hang on, what's the return on this massive investment in British comedy?'"

Webb, reminding us that the two men are a double act both on and off the screen, quickly adds: "It's great you've suddenly got two arts channels, but for how long? Once certain aims and ends have been achieved ..."

Mitchell: "One also fears that the fact that the Murdochs' influence within Sky is not now going to increase, and is therefore potentially reducing, then the interests of Sky in investing ...

Webb: "… in anything other than football ..."

Mitchell: "… might be reduced. I would say, for their balance sheet, that the money spent on football is probably a better investment than that spent on comedy."

While they're not ruling out ever working with Sky, Mitchell says: "It's the sort of thing where you don't know how long it is going to last. Whereas unless the BBC is destroyed ..."

"Or manages to destroy itself out of sheer self-hatred and courtesy," says Webb, who slips into the polite voice of a BBC mandarin: "You really want us to fuck off, don't you? Yes, we are sorry about the licence fee."

Mitchell: "Yes, we are responsible for the entire culture of the 1970s."

Webb: "And for most paedophiles."

Mitchell: "Yes, child molestation came out of a meeting at the BBC in 1972."

Webb: "We were all a bit pissed, sorry."

Mitchell: "Yes, and I'm so sorry it rains sometimes. And it's my piss on your head."

They roar with laughter. If only George Entwistle had made such an apology over the Jimmy Savile/Newsnight affair. In fact, somewhat presciently, given that our interview takes place before Entwistle's departure, Mitchell reckons that the role of the BBC director general is "basically saying sorry all the time".

After almost two years away, Peep Show is returning to Channel 4 this Sunday for its eighth series. The world has changed since we last saw their alter egos, Mark and Jeremy. Dictatorships have fallen and the eurozone has floundered, but the lovable (and sometimes unlovable) Peep Show losers remain locked together in career misery and relationship purgatory in that Croydon flat. Despite feeling rather tired after travelling to a literary festival to promote his new autobiography, Back Story, Mitchell sparks up: "They haven't moved on much, which is sort of vital. Sitcoms often go off the rails when the situation changes too much."

As for his character, Webb says: "Jeremy was going to move out at the end of the last series. You'll not be amazed to hear he doesn't quite do that."

This new series promises "more agony, more pain": office worker Sophie, played by the brilliant Olivia Colman, is missing, but druggy Super Hans is back, and gives Mark a job in a bathroom shop. Meanwhile, Jeremy becomes a life coach. There is also, apparently, a tragedy. Does it concern Mark's baby? Should we prepare for an EastEnders-style cot death? They laugh and wave dismissive hands. "No, no," says Webb. "It's not gritty Bafta territory."

Peep Show, which launched in 2003, has now overtaken Drop the Dead Donkey as Channel 4's longest-running sitcom. Webb reckons the secret to its longevity lies in the fact that its writers, Sam Bain and Jesse Armstrong, resist calls from Channel 4 to knock out each new series quickly. "They work very hard on each episode and the structure of each series – and treat themselves to the jokes at the end. That keeps it fresh and compelling. They've always been very strong and said, 'No, it takes this many months to write a good sitcom.'

"Then there are the characters. Everyone can relate to two people who constantly feel, in different ways, they are in the wrong place at the wrong time. Mark and Jeremy, even though they want very different things, constantly feel they are in this peripheral puddle – stuck with each other while everyone else is having a party."

After meeting at Cambridge university, Mitchell and Webb rose to success via the traditional route of Radio 4 and Channel 4, with a smattering of sketch, panel and comedy shows on the BBC. Unlike their Peep Show counterparts, though, the two have found personal happiness: Webb is married with two daughters; while, at the time of our interview, Mitchell is about to marry the writer and presenter Victoria Coren.

The pair's next step is a BBC2 comedy drama series called Our Men. Due to air next year, it features Mitchell as the British ambassador to a country called Tazbekistan. Webb, who plays his man behind the scenes, describes it in the press release as "Yes, Prime Minister meets Spooks at a bad disco and Yes, Prime Minister is a bit sick on Spooks but Spooks doesn't mind". He wrote that, he says, "after I'd been drinking".

What interests Mitchell about Our Men "is the genuinely funny and dark contradictions in terms of what Britain's representatives are expected to do on our behalf. I think they are expected to achieve the impossible. We like to think of ourselves as a country that promotes liberalism and democracy. But we also like to make sure we've got plenty of money. I think they get a lot of shit if a deal that could be made for the country is fucked up because of an inopportune mention of a human-rights abuse."

Webb is also about to appear in ITV's Marple, in an episode filmed in South Africa over his 40th birthday. So do they want to do more drama? "Not really," says Webb. "I'd like to keep it to every now and then. I can't imagine getting bored with comedy or thinking comedy is beneath us suddenly."

Mitchell, 38, agrees: "You've got to take that attitude. Because some people in drama, incredibly wrong-headedly, have the reverse view. They think, 'You can stop doing this silly little comedy now you're doing a drama.' Thinking of it as a hierarchy is not helpful to anyone – but if there is a hierarchy, for fuck's sake, comedy is better than drama!"

"It's odd because some actors are very scared of comedians," says Webb.

"They're quite right," adds Mitchell, "because you have to be able to make people laugh in comedy, which you don't in drama. It's difficult – they're right to be scared. Obviously, the best drama is brilliant, but which would you destroy: Our Friends in the North or Fawlty Towers? Both amazing shows. It's a hellish world but you've got to destroy one of them. Of course, you'd keep Fawlty Towers."

Webb feels he and Mitchell were "put on this earth" to do sketch shows (or "comedy canapes" as Mitchell calls them) to counter the "very irritating way people can be snobbish" about comedy. But since they're so busy, there's less time for them to watch TV these days, although Webb says he is tuning into Fresh Meat "despite the fact I'm in it", while Mitchell is getting through the 30 Rock box set.

Although he once said Downton Abbey's historical aberrations left him feeling it was just "actors standing there in hats telling lies", Mitchell is now hooked. "I really enjoy watching it. I think it's terrible. I think the script is just ludicrously shit." Webb collapses with laughter as Mitchell warms to his theme: "But it's all very pretty. I like to see a big house and some costumes. And I probably enjoy it as much for the shit script as for the nice performances. But what are they thinking? Have they even read it through? They set up a thing – 'Oh, I think I might have cancer. Oh, I'm really worried.' Next scene – 'You haven't got cancer.' 'Oh phew.' And the whole series about the war. People pop back from the war as if it was the cornershop!"

They would like to write a sitcom together. But as Webb points out: "It's hard to put your heart into it while Peep Show is alive – and of course it's not in our interests to wish Peep Show away because I live in the house Peep Show built. It's been so good to us, and it's such fun. Every now and then, we try to have a meeting, or at least get together in the pub, to think about sitcom ideas. But they're difficult to come up with."

They do have other ambitions: Webb says he would like to play Iago in Othello (I laugh at this, thinking he's joking, but he's not); and Mitchell would like to do a play as well – he's just not sure what one. "Every time I get offered a play," he says, "I'm in a quandary: 'I should do a play, better make sure I do the right play.' I'm in that state of mind at the moment. It feels odd that somehow I got into all this doing plays and reviews at university – and now suddenly, I'm in a world where doing a play would be a weird choice that I'm worried to make. I've got to get through that."

But Peep Show fans should rest easy. As Webb says: "We're never going to be bored with comedy. I can't imagine either of us going, 'We're too grown-up to fall over.'"

• Peep Show returns to Channel 4 on Sunday.