I have copied the following section from this page of the PRS website:

PRS LIVE MUSIC POLICY AND PERFORMANCE RETURNS

The following notes are addressed to performers and/or writers of copyright music. They explain PRS policy for the collection of music details to support the distribution of PRS income from venues licensed for the live performance of copyright music.

How PRS charges royalties for live music

PRS charges licence fees for live performances of copyright music in different ways:

1. Concerts (other than concerts of classical music), where the music is the main business of the promoter of the event and there is a charge for admission, at 3% of receipts from admissions. (This is the rate in PRS Tariff LP, which was set by the Performing Right Tribunal.)
2. Other live music in pubs, clubs, hotels, community centres, and anywhere else apart from the formal concert venues, there are a range of tariffs. These are typically based on a percentage of the promoter's expenditure on music, or a flat rate per night. The licence fee per event is usually far lower than for events charged as concerts. A typical rate for each gig in a small pub would be around £6 (1999/2000 rates).