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“We always use techniques that we’ve never tried before and will never do again,” Jake once told me. “There’s no fun in knowing how to do something; the first thing is to throw away the manual. If you leap from one medium to another, rather than specialising, it dispels the notion of genius. It would be terrible to hover round one material all your life, making work that shows traces of struggle.”

What impact do the brothers want the new work to have? “Originally,” says Dinos, “Jake wanted the sculptures all black, but we painted one to make it more carnivalesque. That’s the problem; what you do at this stage can drastically alter what they are. We’ve always undermined the work, by making it ridiculous in some way, but now there’s a temptation to do something different.”

Dinos isn’t sure whether to paint or patinate the new sculptures. “This always happens. Jake and I start to argue, then finally we agree.” His indecision is a useful clue about how the pair work together. Dinos needs his brother as a sounding board, it seems. Jake has described their partnership as being “as adversarial as possible”. Dinos tells me: “We have an abrasive relationship; like oysters, we need sand to make a pearl.”

Some journalists have found them pretty abrasive, too — an Observer journalist was recently told to “F*** off” when they tired of her questions. Certainly the Chapmans do not suffer fools gladly, particularly Jake. They don’t think you should take their art on the surface. It is packed with art-history references, and they talk about it with intelligence and wit.

On show in Liverpool will be work in a huge range of materials — from drawing and etching to video and wood-carving, and from mud to lead to fibreglass — and each new project demands that they learn a technique well enough to emulate those who are wedded to it; otherwise their efforts would pose no threat to treasured notions of originality, sincerity, commitment and genius.

Even the cardboard and toilet-roll dinosaurs in Hell Sixty Five Million Years BC, have to look authentically Blue Peter. But their work is based on a paradox. They pose as pranksters, but in effect they have transformed painting, sculpture, etching and so forth into performance art.

Seeing their vast output all together, what strikes me, apart from the diversity, is the surprising continuity of their ideas. Amid the deformed limbs and porny insanity, their message is clear: people expect art to be morally uplifting, but in an era when obscene acts of violence are reported on the news every day, the expectation is absurdly unrealistic.

“A retrospective is like a bunch of aesthetic zombies coming to haunt you,” Jake has said. “The worst thing is the linearity, the chronology. We’re trying to undermine the idea of artistic development, the proposition that art evolves towards some kind of perfection.” The retrospective is titled Bad Art for Bad People, because, rather than the art we long for (beautiful, transcendent, idealistic), they give us the art we deserve (which revels in our worst nightmares and exposes our most heinous fantasies).

So they give us a cute painting of two kittens that is too sentimental even for Jeff Koons. Then they go and spoil it by calling the painting Pussy in the Middle, hiding an orifice in the fur and polluting one’s pleasure with questions about tenderness and the infantilisation of women.

Why can’t they be nice, for once? Why didn’t they make me look pretty?

Bad Art for Bad People opens at Tate Liverpool on Dec 15, and runs until March 4 (0151-702 7400, www.tate.org.uk/liverpool)