This is what Wikipedia says of the music played by The Pentangle:
Pentangle are usually characterised as a folk-rock band. Danny Thompson preferred to describe the group as a “folk-jazz band.” John Renbourn also rejected the “folk-rock” categorisation, saying, “One of the worst things you can do to a folk song is inflict a rock beat on it. . . Most of the old songs that I have heard have their own internal rhythm. When we worked on those in the group, Terry Cox worked out his percussion patterns to match the patterns in the songs exactly. In that respect he was the opposite of a folk-rock drummer.” This approach to songs led to the use of unusual time signatures: “Market Song” from Sweet Child moves from 7/4 to 11/4 and 4/4 time, and “Light Flight” from Basket of Light includes sections in 5/8, 7/8 and 6/4.
Writing in The Times,Henry Raynor struggled to characterise their music: “It is not a pop group, not a folk group and not a jazz group, but what it attempts is music which is a synthesis of all these and other styles as well as interesting experiments in each of them individually.”
There is no better example of their unique style than their version of “Jack Orion” on their 1970 album “Cruel Sister”, and I have just discovered an uninterrupted recording of the full 18 minute song on You Tube: