Mattacks established his reputation with Fairport, and all of his session work tricked down from that.
While there was certainly a lot of U.K. traditional/folk activity in the late 50's/early 60's just as there was the post-Weavers phenomenon in the U.S., Fairport Convention, while initially wanting to be a British Jefferson Airplane, kicked off a whole new wave of folk-rock, drawing heavily on centuries-old material, and bringing it to the Fillmore generation. Largely in part from the influence of singer Sandy Denny, who joined after original vocalist Judy Dyble left(she, in turn, was briefly involved in the embryonic King Crimson).
Fairport mingled with the older traditionalists, and had fellow traveller bands like Pentagle and Steeleye Span(who I discovered as an opening act for Jethro Tull). Tull definitely took on influences from Fairport, then there's the entire separate career rabbit hole of their guitarist, Richard Thompson, a 6-string deity in my book. Sandy's solo output as well, although her life ended tragically, but every time you hear Led Zep's "Battle Of Evermore", her voice lives on. The overlooked in his time(and attendant tragedy)of Nick Drake's work is associated with Fairport. The U.K. Cropredy folk festival that has existed for decades is centered on the band.
