Tootler,
This might help keep the bill down when you get your PRS visit:

Make sure everyone who sings (or plays) that night sticks to PUBLIC DOMAIN songs, and makes a list.

Ideally, quote a source book older than 100 years, (e.g. Kidson for Scarborough Fair) or a copy of a broadside, as the origin.

Beware of the copyrighted TRAD stuff like Wild Mountain Thyme
(and Happy Birthday to you). If you must sing it, call it The Braes O' Balquidder by Robert Tannahill, c1795

I think there is a site listing all known public domain songs somewhere in the web (but that may apply only to the USA).

They might get fed up and go away.