One of the founding fathers of free improvisation in Britain. In the mid-'60s, Stevens helped found Spontaneous Music Ensemble (SME), an influential group that included most of England's top experimental jazz musicians. The band's lineup evolved and fluctuated, but at one time or another Paul Rutherford, Trevor Watts, Derek Bailey, Evan Parker, Kenny Wheeler, and Julie Tippetts were among the members. Stevens' father was a tap dancer, a factor in his decision to become a musician. Stevens joined the Royal Air Force in 1958, where he studied music formally and met other like-minded musicians, in particular Watts and Rutherford. While in the service he played with skiffle and Dixieland bands. Stevens' interest in jazz seems to have followed a natural curve; bebop was his first language (he played with hard bop saxophonist Tubby Hayes), followed by the free jazz dialects of the Giuffre-Bley-Swallow trio and Albert Ayler. He became one of the top modern jazz drummers in London during the mid-'60s. He played Ronnie Scott's club regularly, and formed his own septet that included Wheeler.
Around 1965, he joined a group led by Rutherford and Watts. The band's music became more avant-garde than was welcome in regular jazz clubs, so from 1966 they played their free jazz in the Little Theatre Club, a small theater in the West End of London. The Rutherford-Watts group became Spontaneous Music Ensemble. Soon, Wheeler joined the group, then Evan Parker did as well. SME recorded its first album in 1966. By 1967 the band had grown to a septet, with the addition of Bailey and bassist Barry Guy. Stevens' playing became increasingly textural and minimal. At his instigation, the band's music as a whole became more subdued. Except for Parker, the other members dropped out, and by mid-1967 Stevens had become the band's sole leader (eventually Parker joined the ranks of the SME's sometime members). Over the next several years such players as Peter Kowald, Barre Phillips, Maggie Nicols, and Johnny Dyani passed through Stevens-led ensembles. In the late '70s, Stevens used younger players: Nigel Coombes, Roger Smith, and Colin Wood. Wood dropped out in 1978, and the others played (very occasionally) as a trio until 1992. The last SME included Smith and saxophonist John Butcher. The group's final album, A New Distance, was taken from live performances recorded in the year before Stevens' death in 1994.
While SME was probably his most important association, Stevens never stopped playing in other contexts, from rock to bop. At various times he led a large Spontaneous Music Orchestra and the jazz-rock group Away. Stevens died of a heart attack at the age of 54.