Conversation so far:
who developed british folk rock?
Sandy Denny began steering the group towards traditional British music, most notably on the song A Sailor's Life, which laid the groundwork for British folk rock.
what did Fairport Convention do next after the song A Sailor's Life?
Both Denny and Hutchings left before the year's end; the latter replaced by Dave Pegg, who has remained the group's sole consistent member to this day.

Question: what was their greatest achievement?
Self-contained rewrite: what was Fairport Convention's greatest achievement?