John,
Another useful source which attacks half-baked research like that of Harker and Boyes is Chris Bearman's online thesis The English Folk Music Movement 1898-1914. It is extremely thorough and the intro has some useful things to say on folk song definition. Whilst Harker and Boyes present some useful ideas their research is tainted by an overt political slant, and Bearman exposes this cruelly.

Don't get me wrong, my own researches back up much of what Harker has to say on fakesong, but the book is spoilt by overstating any political ideologies of the fakers, and it is full of statements on people like Sharp that do not stand up to the sort of scrutiny Bearman puts in.

Bearman also attacks Thompson on the origins of folk songs, but his research is lacking on this one.

Warning! it is not light reading. (I haven't yet read Thompson's thesis.)