Two further points:

1) Back in the 30s 40s 50s 60s it was very common for pubs and clubs to employ a pianist one or two days a week to lead a sing-song session. There was never any trouble in distinguishing that there was only one performer, but most of the audience would be singing. The obvious point is - where is the line drawn, if only a few people are singing at any one time - are they performing or joining in?

2) While listening to Radio 2 I heard a small item which suggests that the PRS are about to go after people using musical items for pub quiz purposes. Should we set up a formal body to anticipate action from this direction?