I think your idea of a test case is on the right lines. We depend on the goodwill of landlords and it's them not us who get the stick if we fight the case on a specific session/pub. A complaint from Joe Bloggs on the grounds that he is being prevented from hearing his favourite! music could be the way to go. The presumtion is that Joe Bloggs is "passive"- that is he doesn't play sing or do anything that might classify him as a "performer".