In reply to JonmapDL:
I used to do this fairly regularly whilst at university in Exeter.
From the main bus station near Princesshay you can (could?) get a bus to Bovey, from where it's easy enough to walk the rest of the way. It's possible there's a bus up to Widdecombe (close to Bonehill) from Bovey, but I wouldn't know about that
HTH.