The Fairport/Steeleye influence
I got sucked into a history of Jethro Tull while checking on album release dates... it might've been the Wikipedia entry. Anyway, Ian Anderson was close with a lot of the members of Fairport Convention and Steeleye Span, the primary Brit folk-rock bands. He seems to have been as turned on by the music as their other fans, and took his that way.
Steeleye moved to the Chrysalis label, where Anderson/Tull already had gone... and Dave Pegg, the bassist from Fairport, became a member of Tull for a tour or two and a couple of albums. (I saw him on the "Stormwatch" tour).
I should also have included at least one Steeleye album on my list, but I couldn't choose between "Hark the Village Wait" and "Ten Man Mop".