Two unhelpful 'definitions' of folk song:

(i) "I like folk song, I like this song - therefore this song must be folk song".

(ii) Anything in the 'Folk' section of HMV - by this definition Welsh Military Brass bands are folk song.

Personally, I quite like Sharp's idea that to be accorded the label 'folk song' a song must have been through some sort of process involving oral transmission etc. - but there are so many if, buts and maybes that my head starts hurting.

Oh go on then ... if you like it it's a folk song! what's the harm?
I know, let's replace folk music with rock music and call it 'folk music' - now we're all happy aren't we? I and a few other 'purists' are a bit pissed off but 'folk music' is a lot more commercial now and we don't have to think anymore, do we?