Windy referred to the point that "the injustice to which the original poster alluded is the fact that the UK PRS expect their slice regardless of whether the music performed is in'the public domain'or not."

If you can demonstrate that all the music performed is either non-copyright or your own copyright, or that you have the written permission of the copyright owner (unless he/she is a member of PRS, in which case PRS controls the rights)then you can tell PRS to take a running jump. However it is very difficult to be sure, especially in a session or folk club, that none of the material performed at any point is copyright, particularly as many songs or tunes widely assumed to be "traditional" are in fact copyrighted.

The PRS says they can't apportion the licence fee between copyright and non-copyright material, and I have some sympathy - it would be an adminstrative nightmare, and I'd rather they devoted their resources to seeing that composers' royalties are more fairly distributed.

So if you play a single piece of copyrighted music, then a PRS fee is payable - simple as that.