I think a lot of people are getting the wrong end of the stick here. Firstly, PRS licence for the public performance of music as layed out in the 1988 Copyright, designs & patents act.
The act does not specify what is considered a 'public performance', in that it does not say how many people are required to make a performance public. Nor does it highlight the length of time a piece of music has to be played for.
For example, if Jimmy Page walked into a unlicensed music shop where someone was playing 'Stairway...' on guitar, he could (under the aforementioned act) sue the shop owner, (as it is the person responsible for the premises).
I feel that although this is a very difficult situation, (and my personal belief is that the music shops should not require a licence for someone trying out a guitar, etc), the act from which PRS works is not the most detailed, hence these situations.
On another note, I do feel that traditional musicians (as always) are often overlooked by collection societies and perhaps a body should be set up to address this...