I quote from PRS Website
"When music is played in public the owner of the copyright is entitled, by law, to payment from the music user. The Performing Right Society is an organisation that collects and distributes this money (known as 'royalties') to it's members - the owner of the copyright (usually the composer or their publisher).
A PRS licence is required for the live performance or public playing of copyright music by any means. This includes jukeboxes, CD's, video, radio, TV or live bands and discos."
The licence is not it seems specific to each individual peice of music - I doubt if a player in a session will get anything from PRS if s(he) plays an original composition. The quote is from a site encouraging music as a means of attraction to premises!