GUEST said "According to PRS (the bastards) all songs belong to them and there are no traditional songs."

Not true I'm afraid. PRS recognises that there are traditional songs and copyright cannot be claimed for these, although it can be claimed for an arrangement

"Every traditional song sung live is a new arrangement and so has no place in the "trad / arranged" canon so beloved of certain individuals and those bastards at PRS. "

Wrong again. Simply singing it unaccompanied is not an "arrangement". According to PRS's own guidance, for an arrangement to be copyright it has to be a bit more than just putting a few chords to it.   Where an arrangement is sufficiently original to be copyright in itself, it is only the arrangement and not the traditional melody which will attract royalties.

Where the PRS does lay claim to traditional music is in its licensing of venues. They claim that even where venues claim to perform only non-copyright material, it is likely that some of the pieces performed will in fact be copyright. The very existence of this thread shows that it can be difficult to know whether or not a song is really covered by copyright.