Categorising, defining,... hmmm.. yes.. very difficult...

Especially when it almost inevitably leads to'proscribing' !!!

"Almost inevitably"?

My experience doesn't go back very far, but by the same token it is current. I've sung in Hartlepool, Helston and several places in between, including almost all the clubs currently running in Manchester. And I have never, ever, ever seen anyone proscribed, barred, discouraged, criticised or so much as gently sniffed at for bringing a new song to a folk club. It just doesn't happen; maybe it did at one time, but it doesn't now. You're much more likely to be discouraged from doing a long ballad - my local FC actually has a half-serious 'rule' banning songs longer than 20 verses (which would exclude most versions of Musgrave & some of Lord Bateman & Patrick Spens). (Never mind that Lord Bateman taken at a decent pace takes about half the time of Desolation Row or Percy's Song.)

Anyway, I'm not proposing that anyone should change what they do - I don't always sing traditional songs myself (I even write my own sometimes, shock horror). I'm just saying that you shouldn't call something a folk song if it's not what Child, Sharp et al would have called a folk song. But this is such an incredibly straightforward point that the disagreement it provokes obviously isn't motivated by failure to understand it, so I'll stop restating it.