This from Stephen Kellet - posted on uk.music.folk.

Following on from Pete Coe's posting I contacted Barry Lane via email.

I found some of his answers very informative, although they aren't
directly relevant in this discussion. However when pressed about
out-of-copyright and very old music his answers got, I thought, rather
obscure - implying that just about everything was covered (well thats
how I read it) because of modern arrangements or someone would play a
modern tune in the same session. I don't think he was deliberately being
obscure, just answering in the widest possible catch-all manner to make
sure he didn't miss anything. I tried rephrasing the question several
times and eventually reduced the question to the following: (William
Dixon's tunes are from 1733).

Myself:
"If my William Dixon appreciation society were to meet as a session in a
pub and only play William Dixon's tunes would a PRS fee be liable? If it
would, why would it?"

Barry Lane:
"Traditional music in its original form will fall in the Public
Domain (not be protected by copyright). If sessions consist exclusively
of such music, no PRS licence will be required."

Followed by the comment that in the PRS's experience most sessions, folk
festivals etc do include music that is licensable. Etc.

Regarding the "promoter" argument put forth by Dan Plews last year -
that the promoter is liable - that is a red herring - the promoter is
not necessarily liable. The PRS don't need to rely on the promoter to
get enforcement. The copyright act states that the owner of the premises
is responsible for a primary infringement of copyright. The owner need
not be the promoter. This is the principle reason the PRS go for the
public house - because its easier than the musicians (also primary
infringers in cases of copyright infringement) and the law allows it. I
had thought the landlord was possibly guilty of secondary infringement -
Barry clarified this - it is primary infringement.

Barry also stated that any waiver that is in a tune book (such as the
Blowzabella tunebook) does not apply to PRS fees as PRS members have no
right to waive the fees.

Barry Lane:
"I suggest that if any PRS member who believes they can waive
their rights in such circumstances should contact PRS member relations
for clarification. "

My conclusion is that PRS have no interest in ensuring any music
tradition dies or thrives, only collecting fees. Bizarrely a profit
driven organisation may realize that it is in their long-term interests
to except sessions from fees as it is a breeding ground for future
talent. Because PRS is not profit driven, that type of thinking doesn't
apply.

Three solutions:
#1 Hope they don't find where you are playing. Not a very good solution
- especially for the landlord if he gets caught.
#2 Pay the PRS licensing fee.
#3 Make a list of tunes which are guaranteed out of copyright or very
old and ensure the versions you have is the original and not a copyright
arrangement by someone else. The session only plays tunes from this list
(which should contain thousands of tunes).

If doing #3 I guess the "sin bin" will be required to pay the license on
the odd occasion a tune that is copyright does get played. Then you can
play all the modern tunes for the rest of the evening.

I think this PRS thing is very short sighted in terms of the damage done
to grass roots music making. That said, it does appear that the law is
stacked heavily in their favour (except for #3 which they cannot touch).

I hope you find this informative. I am not a PRS member.

Stephen