Is there a new interest in long-neglected Sixties playwright Peter Barnes? Two revivals do not a come-back make, but this production of his 1969 play, following Jamie Lloyd’s James McAvoy-starring  The Ruling Class, is at least putting his name back on people’s lips. And reminding us what a bold playwright Barnes was: his work smashes together big ideas about religion, class, madness and human nature with zinging dialogue, song-and-dance routines, physical madcappery and fleshy rudeness.

Noonday Demons is a chamber piece compared to The Ruling Class’s symphony, but it exhibits many of the same concerns: at their hearts, both plays have a battle over what it means to be good, to be holy. Where The Ruling Class was about an English aristo with God delusions, Noonday Demons takes us into the strange world of Desert Fathers: 4th century Egyptian ascetics who lived in the desert, devoting their lives to prayer, on a diet of olives and dirty water. That this is an absurd, grotesque, and foolish endeavour is never in any doubt. The aim, Barnes wrote, was to write “a comic theatre of contrasting moods and opposites, where everything is simultaneously tragic and ridiculous.”

So here is Jordan Mallory-Skinner as St Eusebius, naked but for a grubby loincloth and chains, covered in sores and attempting to love even the maggots that feed on him. He lives in a cave – low light streams in rather aggressively in Seth Rook Williams’ design – with a massive pile of his own shit. Designer Christopher Hone has created an imposing structure that more resembles a mound of melted Caramac, and I’m not sure why something so sloppy-looking is hard enough to scramble over – but it’s possibly best not to think too hard about the plastic properties of poo.

Eusebius is visited by the devil: Mallory-skinner flicks schizophrenically between the upright, pious monk and a hunched Cockney cheeky-chappy tempter, who offers wealth, women and power. The two are nicely delineated, but Mary Franklin’s production could pick up the pace – because the good stuff starts when a rival hermit arrives. There begins an almighty – and utterly ridiculous – battle.

Jake Curran plays the new contender, St Pior, brilliantly: long and lean, with a burning gaze and deliriously haughty, holier-than-thou attitude. They begin with petulant passive aggression, a territorial dispute over cave space, but things soon escalate. Curran and Mallory-Skinner throw themselves into it with physical aplomb and excellent comic timing, till these raggedy figures are wrestling in the dust. Their fanatical piety, we see, only leads to violence.

Barnes’ mocking of religious extremism isn’t exactly subtle, but you can see why it would appear ripe for restaging right now. And laughter is our own best weapon, he suggests. The one thing that almost tempts Eusebius back to reality? The devil’s jokes. “The world swarms back if I laugh… I wilt not laugh!”

The play can feel dated – the vaudeville double-act shtick and random musical number (‘Monks’, a riff on ‘Kids’ from Bye Bye Birdie) feel of-their-time. The clash of grubby jokes about piles of shit and foul smells with the sanctimonious saintliness probably felt more spicily iconoclastic in the Sixties. Meanwhile the surreal but satiric silliness – a holy water fight; boasts of who has suffered the most (“I prayed ankle-deep in flesh”; “I scourged myself wi’ scorpions”) – often feels derivatively Pythonesque… even if that is a deeply unfair adjective given they are exact contemporaries.

For all that, it is still a striking play, with a singular intensity and rancid energy to both the writing and performances. And Noonday Demons is not just smart-aleck surface: there’s a key moment when it seems Eusebius judges our own time, our own relationship with god – or godlessness.

The two hermits do crap miracles – levitating, transporting – in which they do not appear to move: such fakes! But then, Barnes shifts the territory. Eusebius does, briefly, seem to genuinely travel forward in time, describing a modern city, horrified and haunted by its inhabitants’ cramped disconnection: “They are never alone, never alone with God. ’Tis a vision of hell.” It’s a lovely moment of rug-pulling, an allowance that there might be something bigger and stranger beyond our shitty, earth-crawling lives after all. It doesn’t undermine Barnes’ mocking of religious fanaticism – it salts it, with an awareness that any extremes are potentially corrosive to our humanity.