Alan Moore is best known for his comics Watchmen, V for Vendetta and From Hell, all of which were turned into so-so or (in his opinion) no-no big-studio movies. For me, though, he hit the heights with his 2006 essay Unearthing, a tribute to Steve Moore, who created the UK’s first comics fanzine. Here he describes his mentor’s home in Shooters Hill as part of “dreaming London”, a place where “residues of fossil night-sweat crown the tumulus”, a “life-sump for the Neolithic swill to fill, the pallid Morlock scum”.

This is Alan Moore in full-on psychogeologist mode, envisioning deep time – with its glowering, mesmerising violence – beneath the surface of everything that masquerades as the present day. It gives a V-sign to literary realism, to the twee cosiness of much modern landscape writing. This London is both hyperreal and fugitive, populated by rogues and occult reprobates, littered with the residue of pulp publishing. And the language! Steve isn’t born; rather, he unwinds from “the luminous coelenterate complexity that is his mother”.

In many ways Unearthing reads like an extended prologue to The Great When, the first volume in a planned “Long London Quintet”, its title a pun on pamphleteer William Cobbett’s belief that the early 19th-century English capital was becoming a “wen” – a boil, a sebaceous cyst. It’s set in the year that Steve Moore was born: 1949. London is a bombsite, its streets gappy, its people in a state of PTSD. The war’s been won, but the magic of the metropolis has ebbed. The welfare state embodies a worthy stagnancy, a creeping standardisation. Whither bohemianism?

In this broken interzone lives Dennis Knuckleyard – his name, like those of other characters such as Flabby Harrison and Tolerable John, evoking the world of comics and Bash Street mayhem. He’s 18, orphaned, hoping to be a writer. More than that, he’d love a girlfriend. As it is, he works in a Shoreditch bookshop run by Coffin Ada, who may or may not have been a starlet in her youth, but is now a foul-mouthed, chain-smoking, pink–dressing-gown-clad harridan who would rather rip up a volume than haggle with a customer over its price.

One day Ada sends Dennis to Soho to buy another seller’s stash of titles by Arthur Machen, an author associated with “weird fiction” and beloved by the likes of HP Lovecraft, China Miéville and Mark Fisher. Among the teenager’s purchases is a book that Machen made up for one of his stories – a heretical disquisition about “Heavenly Chaos”, supposedly penned by a Victorian churchman with flaneurial tendencies. He shows it to Ada, who promptly kicks him out and warns him to return it. What diabolical power does it have?

The book leads to terrifying encounters with “Long London”, a mutating, phantasmal place full of “gastropodous chariots” and “colossal glans”. There are shades here of the fever-dream fictions of JG Ballard and Brian Catling, of Thomas De Quincey’s Confessions of an English Opium-Eater, of the metropolitan mysticism of William Blake and Iain Sinclair and the rebel republics invoked by surrealists and situationists. Reality is an asylum, an illusion. This London is, it’s claimed, the “Fire” to the mere “Smoke” of A-Z London: the “Music” rather than the “Echo”.

Animated and anarchic, it also teems with odds, sods and margin-men. There’s Jewish gangster and self-styled King of the Underworld Jack Spot; and the exotically plumed Ras Prince Monolulu – an “auto-legendary racetrack oracle” – who purported to be from Abyssinia, peddled tips at Epsom and anti-baldness cures at Petticoat Lane, and was known for his catchphrases “I gotta horse!” and “Black man for luck!” There’s John Gawsworth, described by critic John Sutherland as an “excessively minor poet” and the literary executor of horror writer MP Shiel, whose ashes he sprinkled on dishes served to dinner guests. Most of all, there’s Austin Osman Spare, esoteric artist and occultist, a reporter’s profile of whom once bore the headline: “Father of surrealism – he’s a Cockney!”

There are set-pieces involving self-stabbing giants which would be catnip for the makers of those mega-budget film adaptations Moore abhors; a romantic subplot too. Key characters reveal themselves as much younger than they initially seem. As narrative, it can at times be stop-start, opting too often for exposition and exegesis. Riffs on theological terms such as “perichoresis” are superfluous. A final chapter set at the end of the 1990s may be a bridge to later volumes in the series, but, with its puzzling allusions to Joe Meek, Prefab Sprout and Margaret Thatcher, is a touch diminuendo.

At his frequent best, though, Moore is, well, more. In Unearthing he railed against Tolkien, CS Lewis, JK Rowling: linguistic disenchanters whose work is “pasteurised for ordinary consumption”. In The Great When, his own language is inflamed, a beautiful riot. Onomatopoeia, rhyming slang, wicked anachronisms, slap-happy metaphors: this is the antidote to the Ozempic prose of modern, MFA-incubated novellas. Sometimes it falls flat, but mostly it’s a sustained feat of bravura brilliance. Bring on the next instalment.