Vic Reeves is blowing into a recorder and improvising a jig, solely to put off Bob Mortimer, who is about to make an entrance. A grinning Matt Berry is also in on the joke. On the other side of the set, a six-foot woman rubs men’s tummies as if they were dogs. Barbara, the warm-up act, gets ready to raffle bottles of Dettol and HP Sauce between takes. Bob makes his entrance in obscenely tiny shorts, tunelessly hammering a cheerful ditty, and collapses into laughter four words into his first line.

“CUT!”

Welcome back to House Of Fools, the latest box of surreality from Vic Reeves and Bob Mortimer. They may be TV’s foremost lunatics, more madcap than the mad hatter’s hat factory, but their sitcom actually has a simple premise – as Reeves explains: “Bob’s trying to keep his house in order, but it’s been penetrated by fools.” Those fools include Beef Stevens, a sexually rapacious colonial Quentin Crisp; Vic’s ex-con brother Bosh; and Erik, Bob’s son by a Norwegian trawlerwoman (joined this series by his near-identical new girlfriend, Rachel). Next door lives upper-class nymphomaniac Julie, who photographs men’s crotches while encouraging them to “buff my Barnaby Rudge”. Strangest of all is Vic Reeves, who’s, well, Vic Reeves. The first series saw the motley crew attempt to photograph a ghost in order to win a chimp, and recreate a pork pie in the hope of bribing Bruce Willis. It’s not The Wire.

No one else could, or would, make a programme like this. From their earliest sketch shows – Vic Reeves Big Night Out, The Smell Of Reeves And Mortimer, and Bang, Bang, It’s Reeves And Mortimer – the pair have been responsible for some of the oddest televised entertainment of the last quarter-century. They’ve given us Morrissey The Consumer Monkey, written songs exploring the link between cottage cheese and voodoo, and asked Michael Winner whether a human could leave fingerprints on a parsnip. In comedy terms, they’ve not so much ploughed their own furrow as taken the tractor for a joyride.

Their dadaist non sequiturs and demented music-hall inclinations made them cult heroes. Always by the side of the mainstream, running the other way, they’re spiritual forebears of The Mighty Boosh, but otherwise out of step with contemporary performers. “I think it’s still an establishment thing,” says Mortimer. “Since the 60s and The Frost Report, people want to be seen as having intellectual opinions. With comics now it feels like you’re meant to leave thinking: ‘Wow, he’s a clever bloke, isn’t he?’ I remember the brilliant thing I’d get from Tommy Cooper was leaving thinking: ‘Fuck, he’s an idiot! What a moron.’”

Despite being the definition of “niche concern”, Vic and Bob’s originality, their taste for the brilliantly stupid, has infected us all. “Funnily enough, Shooting Stars, that stupid little panel show, is the most influential thing we’ve done,” says Mortimer. “You can see it in QI, 8 Out Of 10 Cats Does Countdown, Celebrity Juice… everything’s a bit Shooting Stars now.”

“On Corrie recently, a character said something controversial, and the other one held up a pretend handbag. I see a lot of footballers rubbing their legs,” Reeves adds, with satisfaction. “People don’t know why they’re doing it, really. I think that’s great.”

Well, it’s great unless they’re a rival geordie duo. “Ant & Dec have always nicked stuff off us,” he maintains. “We met their writers, they said they just trawl our stuff and adapt it. The problem is they’re a lot bigger than us, so people think we’re copying them.”

Imitated they might be, but still there’s no one quite like Vic and Bob. Even remotely like them. Outside comedy, the pair have led equally idiosyncratic lives. Reeves is a painter of repute, with an interest in historical mysteries, who’s had a No 1 single (1991’s Dizzy, with the Wonder Stuff). Mortimer is a former punk and a former solicitor (who famously represented Jarvis Cocker when he was arrested for arse-flapping Michael Jackson at the 96 Brit awards). Basically, it takes brains to play it this dumb.

“There are quite detailed rules with sitcom,” Bob explains when we catch up with the pair in a BBC conference room a few days later. “When people can leave scenes, act structure, joke rhythm. You can’t not have a straight man… although I’m not entirely straight.”

You’re not entirely straight?

“He’s a vague anchor,” reveals Vic.

“I desperately need these people around because I’m a sad bastard,” says Bob, picking up the thread. “I have a son I want to love but can’t because he’s horrible. That relationship’s not just idiot-idiot, it has heart. Julie’s done a load of laudanum, and seems cheerful in that indulged, posh way; but why is she hanging around with us? There’s the sadness.”

The girl next door: Morgana Robinson as nympho neighbour Julie. Photograph: Christopher Baines

Another thing about House Of Fools is that it’s a multi-camera, live-filmed sitcom, of the type popular when TV was black-and-white; a 50s throwback, if you like. “We use old-fashioned techniques and play about with them,” says Vic. That’s a bit of an understatement. The cast are always prodding at the fourth wall: acknowledging the cameras, the audience, pointing up the farcical mechanics of their story while cracking on with it anyway (“Where are you going, Bob?” “Just… to move the story on a bit.”)

In the dinner break halfway through recording, I go in search of their supporting cast. They’ve always attracted talented misfits, and House Of Fools is frankly an oddbod allstars. At one table, Dan Skinner and Daniel Simonsen are sitting next to each other, as if a register is about to be called. Skinner is familiar as Angelos Epithemiou from Shooting Stars. In the first series, Bosh was a perma-angry, camo-wearing geordie Wolverine; now he’s wearing a side parting and blue polo neck, like a fifth-rate Roger Moore. “I decided he’s been to a cult and now wants to be on CBBC,” is the explanation Skinner offers. Norwegian comic Simonsen plays Erik, with an exaggerated accent that sounds like a confused, sarcastic sea lion. (“We worried that no one would understand what he was saying… but that doesn’t matter.”)

Beef and Julie, AKA Matt Berry and Morgana Robinson, sit nearby. Berry has been a recurring presence in the funniest shows of the last decade, from Garth Marenghi’s Darkplace to The IT Crowd. I’ve previously met him for an article on his own superb sitcom, Toast Of London. “You spelled my name wrong. Was that on purpose? Where’s your mic?” he says, parodying the foul-mouthed bluntness of many of his characters, I hope. How did he get involved with the show? “Jim approached me,” Berry explains (using Reeves’s real name, Jim Moir). “He didn’t know what it was going to be about; he only had a name, Fuck This House. Of course I wanted to be involved with that.”

Fool house: Matt Berry as Beef. Photograph: Chris Baines

I note the anarchic energy on set, the semi-improvised tomfoolery. There’s almost a liberating amateurishness, I suggest. “HOW DARE YOU. Who the fuck is this guy? I think what you’re trying to say,” he gently corrects me, “is it’s done in Vic and Bob’s style, which is unique. I don’t look up to many comedy people. To be in their sitcom – it doesn’t get better than that.” Robinson looks up from her salad, and agrees succinctly: “Of everyone, they’re the coolest.”

So you’ve got brilliant friends, hidden depths, and Bruce Willis played by a fig in a vest; but it’d be nothing without the two pals at the heart of it all. Vic and Bob’s chemistry embodies the improv principle of acceptance: never blocking, open to every idea, building on each other. It also makes them fantastic off-the-cuff interviewees. So I ask them what their street weapon of choice would be.

“Can I have an animal? Well, I’m sorry but I’ll have a bear then,” replies Bob without hesitation.

“You’d carry an attack bear around with you? On a chain?” Vic smiles.

“A bit armoured, yeah.”

“That’s hard to top. Maybe I’d have a giant pelican, so I could dissolve my enemies in its beak pouch.”

Back on set, everyone’s horsing around like it’s the last day of term. Robinson winks and wiggles for the crew, who are clearly in love, cameras trailing her like ducklings. Skinner tries out comic variations, switching accents, bits of physical comedy. Vic sings Footloose while dancing like a lunatic, for his own amusement. Matt Berry is laughing. The cameras roll and… Bob forgets his lines.

“I do sometimes wonder what we’re doing, two middle-aged men talking about breadstick hands and having no genitals,” Bob wonders aloud. “But then you come up with an idea for a whip that can stop time, and you’re up and running again.”

House Of Fools starts Monday, 10pm, BBC2

This article was amended on Saturday 14 February 2015 as it originally referred to Vic and Bob as Geordie, when in actuality Vic Reeves is from Darlington and Bob Mortimer is from Middlesbrough.