Martin Carthy certainly used guitar extremely skillfully. When did he first come on the scene? (I'm pretty ignorant) Were there any before him? ===

Yes, afraid you are, Ian. Answer is late-50s,& there were literally 100s before Martin; he was, as you say, exceptionally skilful, & introduced several new tunings & single-string techniques, &c; but basically he simply jumped on an already rushing bandwagon ~~ long preceded by Burl Ives [far more influential guitar-wise than Seeger]; by the skifflers; by the lady who sang to Cecil Sharp in the Appalachians with guitar accompt in the 20s (one of the few genuine traditional accompd singers); & by Elton Hayes over here, who appeared about same time as early Ives ~~ ("Hayes took up the guitar shortly before World War II when he accepted one as security from a friend who had borrowed 30 shillings. Guitars would cause his later fame, accompanying him while he sang old English folk songs and ballads" [Wikipedia]). Then, also pre-Carthy, there were Henry Morris, the McEwen brothers Rory & Alex, Nancy Whiskey, Robin Hall, Jimmie McGregor, Steve Benbow ... all beat Carthy to it by at least 2 & up to 5 years.