Roy Harper is the best singer/songwriter that you aren't listening to; don't ask me how I know that you're not listening to him, but I know you're not.
Now you can. Here's two of his records from 1970 and '71 respectively; Flat Baroque and Berserk & Stormcock. One is 12 songs recorded in a more conventional format with lengths ranging between a minute-and-a-half and eight-plus minutes, with some straight-ahead Brit-folk interspersed with contemporary sounding rock and blues. The other is a sprawling, massive four-suite prog folk masterpiece.
Harper was name-checked by Led Zeppelin on the track Hats Off To (Roy Harper) on their third album and also the featured vocalist on Pink Floyd's Have a Cigar from Wish You Were Here. So he's your favorite musician's favorite musician. So you should be listening to him. A lot...