A new double CD by the Joseph Holbrooke Trio, “The Moat Recordings” (Tzadik), has been both seven and 43 years in the making.

Named for the early 20th-century British composer, the trio of the late guitarist Derek Bailey, bassist Gavin Bryars, and drummer Tony Oxley formed in 1963 in Sheffield, England. They started off playing jazz standards, moved into modal jazz territory, and by 1965 had evolved beyond jazz into what became known as non-idiomatic (or “free”) improvisation. But after making a clutch of recordings, virtually all of which have been lost, Mr. Bryars abandoned the group – and dropped the bass altogether – in 1966.

The three did not meet again until a reunion concert in Cologne in 1998. The following year, a recording session was arranged at Bailey’s preferred studio, Moat, and Gary Todd produced it for his Cortical Foundation label. The release was held up by an accident that left Todd incapacitated, and was further delayed by Bailey’s battle with motor neuron disease (he passed away last December). Now, John Zorn’s Tzadik label has finally managed to bring the sessions out. Aside from a live recording of the reunion concert and a short rehearsal excerpt released a few years ago, this is the only recording of this collaboration.

The non-idiomatic improvisational genre is informed by John Cage’s chance-operation compositions, the delicate dissonance of Anton Webern’s music, and the free jazz of Ornette Coleman, Albert Ayler, and Sun Ra. Musicians play with no predetermined structure, unlike in free jazz, which still includes a melody at the beginning and end of a piece. The form started in several circles throughout Europe in the mid- to late 1960s; its original exponents included Bailey, Mr. Oxley, and Evan Parker in Britain, Han Bennink and Misha Mengelberg in the Netherlands, and Peter Brotzmann and Alex von Schlippenbach in Germany. In the 1970s and 1980s, it gained a foothold in the United States with younger musicians like Mr. Zorn and Eugene Chadbourne.

The Joseph Holbrooke Trio was completely unknown outside Sheffield in the 1960s, and for three decades remained little more than a legend to experimental music aficionados. Since the mid-1990s, all three members’ profiles have risen considerably.

Mr. Bryars is the best known: After leaving the group to pursue composition, he became famous among the European Minimalists for such singularly lovely pieces as “The Sinking of the Titanic” and “Jesus’ Blood Never Failed Me Yet.” Bailey and Mr. Oxley continued working as players and improvisers. Mr. Oxley played with such free jazz titans as Cecil Taylor and Bill Dixon, while Bailey’s trailblazing spirit led him to collaborate with performers as disparate as pop-jazzer Pat Metheny, drum-and-bass DJs Ninj and Casey Rice, avant-prog rockers the Ruins, and tap dancer Will Gaines.

In 1970, Bailey founded a record label, Incus, with Messrs. Oxley and Parker to document the development of improvisational music; in 1977, he started Company Week, an annual concert series that brought together improvisers from all over the globe. Bailey literally wrote the book on improvisation: His 1980 “Improvisation: Its Nature and Practice in Music” is well worth seeking out.

“The Moat Recordings” offers an intriguing dose of free improvisation in its classic mode – no laptops in sight. Bailey and Mr. Oxley interact with lightning agility – unsurprising given their long engagement with such activity and with each other in the intervening years – while Mr. Bryars’s playing is slower and more considered. This may have as much to do with his compositional style as with his long absence from improvised situations, but Bailey recalls in his book that even in the 1960s Mr. Bryars had a “somewhat ambivalent attitude toward the group, never sure if he should be there at all.”

Indeed, Mr. Bryars’s semi-outsider status gives the music an interesting tension. Sometimes leaning on long, sustained tones, Mr. Bryars brings a certain measure of tonality that is usually missing in the discordant improvisation scene. In his liner notes, Mr. Bryars recalls that Bailey said “he had not had to consider pitch in such detail for a very long time.” Having long since augmented a traditional drum kit with various sound-making objects, Mr. Oxley strokes all manner of metals, producing pitches with post-Cage smears and scrapes that match Bailey’s harmonics with eerie precision.

Playing acoustic and electric guitar, Bailey demonstrates his trademark redefinitions of the instrument; he discards chords and scales in favor of a wholly personalized integration of clusters, single notes, plucking behind the bridge, and other ordinarily extraneous noises. To forward-thinking instrumentalists, he was as important a liberator as Picasso was to painters.

The latter half of the first CD contains several marginally frantic pieces, yet the bulk of the material on the two discs (15 tracks in all) is given over to what Mr. Bryars once termed “the serial equivalent of a free jazz ballad.” These are marked by their spaciousness, and a languor that’s paradoxical considering the variety and second-to-second invention involved and on display.

While each piece has a beginning, middle, and end, there is seldom, if ever, a narrative arc. The results may strike some listeners as almost existential, but they’re also vigorously exploratory, unfailingly spontaneous, and fiercely imaginative. Running well over two hours, “The Moat Recordings” are a long haul but should satisfy both hard-core devotees and those whose curiosity is whetted by the members’ reputations.