He's used a number of tunings. He used to mainly use DADEAE but more recently seems to have settled on CGCDGA (you'll need heavy gauge strings for that)
There's Martin's excellent song book "Martin Carthy: A Guitar in Folk Music" which gives the tunings to many of his best known songs, and where he explains how his playing evolved to using particular tunings. The book also has tab for some tunes.
Also there's a video which although very cheaply produced is excellent, and comes with a booklet of tabs. Can't remember the title offhand but a search at somewhere like Amazon should find it.
Is there a particular song you want to play?