Sod it I'm going to preach again.Once a song has been sung,particularly if its been recorded or broadcast,it has to be in the public domain.Therefore for performance that does not involve monetary gain,any thing goes.Once your making money,the writer deserves his cut,so put in your returns to the PRS.However the stealing of a song,by which I mean using some one elses arrangement and treatment is NOT in my opinion right.By all means use other peoples material,but do it YOUR way,so that it has the stamp of You,not the person you got the song from,and always credit your sources,wether contempory or traditional.Politeness does mean that you leave other peoples stuff alone at sessions they attend,and,if you learnt the song from them,credit them as source at other sessions.