Lets have another go at this as well:

"PS If your songs are folk songs they are public domain by law as I understand it - if they are not why?"

This perfectly illustrates the bassackwards way you are viewing this.

My songs are folk songs because a sufficient number of people call them so.

(I NEVER do - but i allow others to if they want to because they may. I do call myself a folk musician, because i do play both folk and traditional music).

Folk has NO meaning in law. It does NOT mean 'public domain.' It is never used by PRS or any other copyright licensing body.

Traditional DOES have meaning in law. It DOES mean 'public domain.' It IS used by PRS and all other copyright licensing bodies.

Therefore. If I labelled my songs as Traditional I WOULD be telling the world they were in the public domain.

But I don't. Ever.

I let other people call them Folk because they want to, and this has absolutely no impact on their copyright status.

Understand that and the rest might finally fall into place.