One thing I seldom hear mentioned much, is that Dave Mattacks, to me at least, is the first person I ever heard on either side of The Pond that ever found a way make a drum kit fit into a UK or US Folk context and make them sound like they actually belonged there and felt relatively at home (other than in some dance oriented traditional contexts). Drums tacked on to a Bluegrass or folk recording by a producer who seemed to be on some sort of misguided attempt to make the music sound more "commercially appealing" usually made me cringe, i.e. the later Flatt & Scruggs recordings. "Liege" & "Full House" era Fairport (actually encountered in reverse order after a year's exile in Vietmam)made me say to myself, "That's cool!", while I thought that, while they'd used an occaisional session player on their first album, Steeleye was doing just fine before they added the full time drummer for "Now We Are Six" and that their rhythm section sounded a bit on the overly bouncy side a lot of the time, and a bit of a distraction. Mattacks could find ways to add to things beyond merely keeping time and even in later times could make something like "Polly On The Shore" sound like something that I'd have rather heard behind Steeleye's vocal prowess that what they had going, for the most part.
The flexibility of the Fairport rhythm section was a secret weapon for them apart from the ever changing parade of singers, songwriters and lead virtuosoty on guitar and fiddle.