The rule should always be don't provide accompaniment to an unaccompanied singer unless it's asked for. And then agree a key with the singer beforehand and let the singer set the pace. I accompany myself on bouzouki or melodeon, so why would someone assume that they should use an instrument if I'm singing unaccompanied?

Many so-called accompanists are not up to the job anyway. If they can only thrash out three chords, or play the tune that the singer is singing then they should shut up, and it would help if others in the session gently pointed that out to them. Most people are quite amenable if approached in the right way. I tend to only join in with friends I've played with before, or with stuff I know well enough. If Maggie or I don't mind a bit of extra accompaniment we'll say something like 'this one's in D'.

The original 'rudeness' took that started this thread took place in the Endeavour in Whitby. This is not an organised session, but of the type we usually call a 'dive-in' session, in which there is no organiser and the next performer 'dives-in' when the last song/tune is finished. It's often a noisy pub with lots of instruments and people wanting to thrash out quite noisy stuff. We spent a night in there (Monday?) and ended up sort of running it, going round the room and trying to give everyone a chance, which is the only way really if there are unaccompanied singers wanting to do a nice quiet song.