There was a variant of this in the Eight Bells in Tenterden Fest on Friday.

True, there was a drunk squaddie covered in tattoos who was intent on very loud conversation (in the midle of the players' circle) and even jogging guitarists' arms as they tried to play (their turn, not joining in with others), so we were all quite appreciative when a Steward loudly interrupted between two songs and asked people to listen not toalk, or if intent on talking, to use the other bar.

***k me she then walked to the bar (the singer's side) and proceeded to have a very loud conversation indeed, in the immediate vicinity of a rather loud-voiced unaccompanied singer who was also conversing (but has been known to get tetchy if himself talked over). This rose to a bit of a crescendo in the middle of a friend's superb (and I mean that) rendition of "Coal Town Road". I was (by agreement) accompanying his guitar playing and singing, on mandolin. First I stood and took one pace towards the offenders and stared with basilisk intensity - to no avail. At the end of the verse said friend interrupted the bridge, stared right, yelled "shut it" with great intensity, and then returned to the song (as I frantically tried to recover the rhythm of what I was playing along to him).

It was partly successful. But the squaddie continued although the guitarist he was upsetting most left.

Later in the evening the booked host of the session/singaround, an imposing figure of a man, sang one verse of a song, stopped, and started the "if it's so interesting why don't you tell the rest of us about it" routine. This produced embarrassment in the said squaddie and some improvement.

But to come back to the original question, surely the root of the determination lies in the nature of the session. If it was a participative session (whether song or tune) then Raggytash by seeking to exclude participation (either by pre-announcement, or by spontaneous combustion) is at fault - but if it was a "take turns" session, then the intervener was at fault in joining in if not invited.

Problems can arise if the host has said that it's participative, but it is being treated like a rotating concert.