This riff is based on the Free classic "Walk In My Shadow," which Joe B. covered on his Shepherds Bush Empire record. Free is one of the great British blues rock bands that don't get the attention they should. Paul Rogers is truly one of the finest rock singers of all time...I don't think that's in dispute, but I just wanted to say it!
Free guitarist Paul Kossoff died young due to drug abuse, however still wrote the song "All Right Now" and created some of the coolest blues rock riffs in the genre. His lead guitar work influenced many players including Dave Murray and Joe Bonamassa (I can definitely hear a similarity to Kossoff's vibrato and Joe Bonamassa's). Like most bands of the blues rock genre, it's best to listen to the live recordings, so watch some of the great footage of the band on YouTube.