Not long before he died in 2004, Paul Foot wrote an obituary of his friend Tony Cliff, with whom he worked for many years on the Socialist Worker. “Of all the awful crimes of the left, none infuriated Cliff like passivity. For people who knew the world was rotten, to sit back and do nothing about it was for him the ultimate aberration.”

Foot, a prolific writer for nearly half a century for the Daily Mirror, Private Eye, Socialist Worker and latterly the Guardian, wasn’t guilty of that aberration either. Margaret Renn’s biography reveals the posh old Trot hack with the dodgy hair to be something we desperately need but arguably lack today: an unbiddable socialist journalist adept at investigating stories about ordinary people fitted up for crimes they didn’t commit, and implacable in indicting governments for getting their snouts in the trough while the people they represent are systematically betrayed. Foot was, Guardian readers will recall, especially good on New Labour’s ruin of public housing and hospitals via the private finance initiative.

All that said, he was also a symptom of the rottenness he indicted. This former Oxford Union president exploited the old boy network to get ahead. His father, Hugh, had been – if this isn’t too Marxist a description – a colonial lackey, serving as governor of both Jamaica and Cyprus in the dog days of the British empire. Foot parlayed Daddy’s influence with the managing director of Mirror Group Newspapers, Hugh Cudlipp, in order to get a job. He wrote some of his best investigative material at Private Eye, whose Soho offices in the 1960s sound like a gentlemen’s club, albeit for misfits like Willie Rushton and Richard Ingrams. Renn, in an otherwise impeccably lively and well narrated biography, swerves this problem: yes, one might see the case of Paul Foot as typifying the kind of organic intellectual that revolutionary politics needs; or one might see him as a virtue-signalling parasite with no real skin in the game of class struggle.

Even if I sometimes cleave to the latter, I still can’t help but admire his reporting on police corruption in the investigation into the racist murder of Stephen Lawrence; on sexual abuse in children’s homes; on why Norman Scott’s dog was shot; on the dubious rationale for sinking the Belgrano during the Falklands war; on who was behind the Lockerbie bombing and why.

Renn, using interviews with friends and family, paints a portrait of an adorable scourge of corrupt politicians and dodgy businessmen. She doesn’t really account for how he managed to work for one of the dodgiest of those businessmen, namely the Daily Mirror proprietor Robert Maxwell, for so long.

Neither do we get to the heart of why Foot became so implacably, ardently and impressively committed to socialist revolution, though perhaps one reason might be his lifelong fondness for the work of the Romantic poet Percy Bysshe Shelley, who attended the same Oxford college and whose revolutionary verse Foot lifted from obscurity in Red Shelley (1981). In the poet’s Prometheus Unbound, Foot tells us, the subversive figure of resistance is chained to a rock for all eternity for defying the tyrant Jupiter. Until, that is, a monster called Demogorgon, representing the power of the working class, is roused to fight for the rebel Prometheus’s liberation from his cruel fate.

Foot’s gloss on this text expresses the cause he spent his working life fighting for. He wrote: “The only power which could not be contained was the power of the people, organised, united and confident in revolutionary action.”

Paul Foot: A Life in Politics by Margaret Renn is published by Verso (£30). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply.