How do you follow up a documentary that plays out like a tragi-comedy? Easy: make a tragi-comedy that masquerades as a documentary.

After the brilliant Lost In La Mancha, a hair-raising account of Terry Gilliam's aborted efforts to film The Man Who Killed Don Quixote, directors Keith Fulton and Louis Pepe return with another tale of industry implosion - though this time the industry under the spotlight is the music business of the mid-1970s, characterised by pan-sexual hijinks, druggy indulgence, a burgeoning punk scene and, we're invited to believe, the most jaw-dropping double act of the decade. And that includes Bernie Winters and Schnorbitz.

Unfortunately, Brothers Of The Head, suffused throughout with a sense of impending doom, is on a hiding to nothing: a cursory glance at the two previous attempts to adapt Brian Aldiss's works for the screen - Frankenstein Unbound, and Artificial Intelligence: AI - should have sounded klaxons. And, though no fault of his own, screenwriter Tony Grisoni's CV doesn't immediately inspire confidence; neither Fear And Loathing In Las Vegas or Tideland received the smoothest of rides, critically or commercially. Garnish this strange brew with a latterly unbankable director (Ken Russell, cameoing as himself) and it begins to look like deliberate self-sabotage.

Conjoined twins Tom and Barry Howe (real-life identical siblings Harry and Luke Treadaway) grow up in an isolated cottage on the windswept shores of L'Estrange Head, on England's east coast. The twins are joined at the chest by a Cronenberg-esquire protuberance, and it's pretty much taken as read that a surgical procedure would more than likely kill them.

When they hit 18, their father sells them to ailing music impresario Zak Bedderwick (Addfield). Recognising a lucrative novelty act when he sees one, Bedderwick wastes no time in installing the pretty things in his country estate and grooming them for stardom. "Hand on heart, I never exploited anyone who didn't want to be exploited" says Bedderwick. He's all heart.

Under the tutelage of musician Paul Day (Dick) and the strong-arm tactics of their manager Nick Sidney (Harris) the pair are transformed into a fearsome live act, developing drug habits and egos the size and shape of Tower Bridge. Things get messier when journalist Laura Ashworth (Emery, Kent) falls for the gentle Tom ("If you're in trouble and you need a friend, Laura Ashworth is the last person you want coming round the corner," reflects Paul), as the more volatile Barry is caught in a jealous, voyeuristic bind. Splitting up due to 'creative differences' is not an option. It's going to end badly.

With echoes of Hedwig And The Angry Inch, Velvet Goldmine and Peter Watkins' Privilege, Brothers Of The Head rankles so much because there's the potential for a similarly good film here. The music, bridging the gap between glam and punk, is marvellous, if suspiciously redolent of the more recent New Wave of New Wave (of New Wave) style, and the period documentary footage is terrifically authentic. The Treadaways make a good-looking team; their incessant huggy embrace both an act of physical necessity and a sly nod to the era's gender transgressions. Mick Ronson and David Bowie performing 'Starman' on 'Top Of the Pops' springs immediately to mind.

Yet for all their supposed turbulent symbiosis, what we mostly get is Tom looking glum, and Barry acting up. There's no real sense of their inner lives, and the film relies far too much on peripheral characters, such as a surgeon popping up to tell us helpful stuff like "the emotional and intellectual lives of conjoined twins are combined in a highly intricate way." Well, duh.

Toward the end, the film all but fizzles out like a faulty amp, while the belated revelation that Barry may or may not have harboured the remains of a third foetus in his cranium goes absolutely nowhere. Although Ken Russell gets some mileage out of it during a characteristically over-the-top sequence in his pretend-film-within-a-pretend-film, the Howes biopic 'Two-Way-Romeo'. "I think Ken Russell should stick to Women In Love," muses documentary-maker Eddie Pasqau (Bower) wryly, in one of the few genuinely funny lines.

There's a hollow ring to this pretentious picture; a surface film about surfaces. The instinctive action at credits up is a shrug: is that it?