I think you're essentially right George - any public entertainment anywhere requires a Public Entertainment Licence, unless there is a specific exemption. The only exemptions I am aware of are for church services, and for pubs, where there is a limited exepmtion covering two-in-a-bar. (Elsewhere even one person providing "Public Entertainment in principle means a Public Entertainment Licence is needed.)

There does not seem to be any clear or agreed definition of what "Public Entertainment" consists of. That means that it is open to a local council to call virtually any activity "Public Entertainment" and see if it stands up in court. That is where sessions come in. The fact that an activity is traditional or has always been accepted does not seem to carry any significant weight in court, though I suspect that it could, if the justices thought it should.

The other factor is that human rights legislation enters into the picture now, and it seems improbable that a court taking this into account would see it as legal to forbid people to sing or play musical instruments for their own enjoyment, so long as this did not cause inconvenience to other people.