A few years ago, the film-maker Adam Curtis unearthed two alternately bleak and inadvertently hilarious BBC documentaries about Hells Angels. One was a 1973 film positing them as a terrifying threat to Britain, a sentiment slightly undercut as the film progresses by the discovery that their vice-president “Mad” John and his sergeant-at-arms, cross-eyed Karl, can’t even successfully organise a weekend break on a barge near Aylesbury, let alone the destruction of society as we know it. The other was a 1969 doc following the fortunes of the sweet-natured but implausibly dense Sylvia and her delectable boyfriend, Paul, or as he prefers to be known, Hitler. We see their Hells Angels wedding ceremony in a Birmingham pub car park, learn of her new husband’s enlightened attitude to marital relationships – “If she’s naughty, I’ll thump ’er yead in” – and meet Sylvia’s parents: her mother uncomprehending, her father merely incomprehensible, thanks to a Brummie accent that makes Hitler/Paul sound like Princess Anne.

The two films are soundtracked only by horrified narration, but the music on I’m a Freak Baby would have provided the perfect accompaniment. It sounds the way the documentaries look: grim, grey and possessed of an almost wilful ugliness, the latter evident everywhere in sound – oppressive distorted guitars; rhythms that either lumber groggily along or charge at you with eyes bulging, as if they’ve been at “Mad” John’s “pep pills”; vocals that growl, bellow, rasp nasally in post-Dylan style or occasionally bleat like a goat. Even the bands’ names sound as if they have come from the films: a warm welcome, please, for Hellmet, Crushed Butler, Sweet Slag and, perhaps best of all, Bum – alas persuaded by their record company to rechristen themselves Iron Maiden (no, not that one).

In addition, you suspect cross-eyed Karl and Hitler/Paul might have had a lot of time for the bands collected across these three CDs, with their “heavy” music and their proudly touted bad attitudes – a band from Surrey called the Velvet Frogs claimed to be “delinquency set to music”; John Peel favourites Stack Waddy released an album called, amazingly, Bugger Off – and their ability to cause widespread revulsion among genteel society. The sleeve notes are packed with horrified reviews from the contemporary music press: “essentially joyless”, “sordid, squalid … the bottom of the bin”, “from the first note, you know you don’t want to hear any more”.

You can see why these bands might have got short shrift at the time. In an era packed with the more cerebral pleasures of prog rock and self-examining singer-songwriters, what price a bunch of herberts like east London’s Egor, with their 16-year-old guitarist and combover-sporting thirtysomething singer, barking out Street, a hyper-distorted slab of downer blues-rock with an off-key harmonica solo? Indeed, in some quarters, a bad critical rep still clings to this stuff: it’s derided as the stunted idiot offspring of psychedelia and the late 60s British blues boom, with all the nuance, invention and virtuosity of those genres buried in a quest for volume and “heaviness”.

But that’s unfair. I’m a Freak Baby’s four hours seem to be teeming with ideas. Some of these ideas are, admittedly, ludicrous. Sam Gopal had a youthful Lemmy on vocals, wailing bad-trip guitar solos and the kind of grinding, simple riffs that their frontman would one day make great capital from, but no drummer, just a bloke playing tablas. The overall effect is ridiculous. Others, however, are really interesting. Third World War thought rudimentary ur-metal was the perfect vehicle for streetfighting Marxism. Gun’s 1968 single Race With the Devil – one of a scattering of tracks here that made any commercial impact – stirred a blaring horn section into the cocktail. The Open Mind bizarrely hit on a sound you would describe as heavily influenced by the first Stooges album, had their single not been released the same week as the first Stooges album.

And furthermore, this music perfectly captured its era. If one response to the end of the 60s party was the wistful morning-after melancholy found on Simon and Garfunkel’s Bridge Over Troubled Water or Crosby, Stills and Nash’s debut album, then Egor or Factory or Barnabus offer another: music that feels like it’s crashing angrily around with the kind of monstrous hangover that resembles a cross between a profound existential crisis and an actual nervous breakdown, that comes suffused with feelings of failure and self-hatred and the creeping belief that, as Kingsley Amis once put it, “your friends and family are leagued in a conspiracy of barely maintained silence about what a shit you are”.

There are a handful of famous names here – the Groundhogs, Deep Purple and Hawkwind among them – while at least one contributor went on to become about as famous and celebrated as it’s possible to become. On the Yardbirds’ Think About It, you can hear Jimmy Page trying out a guitar solo he’d later deploy on Led Zeppelin’s Dazed and Confused – its impact slightly dulled by the fact that Think About It’s main riff sounds distractingly similar to the tune of I’ve Got a Lovely Bunch Of Coconuts. But it’s not just perversity that might lead you to find the most obscure names here the most fascinating: taped while thrashing away in their rehearsal rooms or in the cheapest demo studios available, the primitive recording conditions add a rawness and a compelling edge of hysteria to Wicked Lady’s I’m a Freak or Iron Claw’s awesome Skullcrusher.

A few years later, the predominant style found on I’m a Freak Baby would be streamlined, warped and codified into heavy metal. Others would find their influence erupting elsewhere. In north London, a Pink Fairies fan called John Lydon was clearly paying close attention to the band’s anthem Do It. But these bands’ real spiritual successors would emerge much later, on the other side of the Atlantic. If someone told you that Jerusalem’s Primitive Man or Little Free Rock’s Dream had actually been released not in 1972 or 1969 but on Sub Pop, around the time of Nirvana’s debut album, you wouldn’t bat an eyelid. In the wake of Nevermind, they might even have been huge. As it was, they sank without trace, and it’s hard not to be pleased that someone’s dredged them up.