Sorry, Howard - yes, I see what you mean. Agreed it's not a very perfect system in terms of copyright material being performed at sessions with no sample being taken to get the royalty to the right writers (but not arrangers, by definition).

PRS do accept this, and they suggest that they redress the balance using the PRS Foundation, with funds festivals and other good works - including some in the folk field.

I think they could do more, and once tried to suggest a fairer system, through the FRATS sections of the MU (who I support). But they'd only recently fought hard to get the current system in place, and as it's a huge improvement on its predecessor my suggestions rather fell on deaf ears.

One solution would be for one member of the session - someone who has, perhaps, put out a CD or a song/tune book (i.e. published some arrangements) - to join PRS and then regularly submit set lists to GACS as 'musical director' of the session. The royalty could then be used to buy a pint and a bag of crisps to share between all the players!

And yes, the PRS website IS one of the worst I have ever encountered, from a design point of view, and I've told them that too - lots of times!

The key is in this phrase from the notes on the DS tarrif in your link:

"repertoire means all and any musical works (including any
words associated therewith), the right of public performance in
which is controlled by the Performing Right Society Limited"

That does NOT include public domain material.