Anne, I have to make a small correction to the above, before the argument strays: You say "if (my songs) feature in someone's repertoire as a guest (when they are being paid a fee) then I would hope the royalties would come my way".

In this case it is immaterial if the performer is being paid or not. The issue is whether the performances are of commercial benefit to the venue. That's why it is the venue, and not the performer, that gets licensed (that also answers the inevitable perennial query about busking). So PRS would argue that a regular session at a pub attracts business, therefore a license is due. An impromptu session would not, however.