Its a dilemma Raggy, sorry I arived too late to witness the event yesterday. Anyway, I have given it some thought and this is what I think.......

Yes, anounce up front that you want accompaniment or not..........if after that you are then faced with something that becomes seriously off putting, especially if a quiet and subtle request has been ignored, then stop..........be honest..........say you are finding it really difficult to play/sing with the offending player/instrument and say you would like to continue without it please. And do all that with a smile of genuine warmth........oh.........and also dont forget................

to make sure you dont knock over the pint belonging to the big bloke sat in front of you with your guitar neck, which verse you are singing, how to play the upcomming guitar bit that you always get wrong, what the next word is, what the next line actually means, where you are supposed to take your next breath, that you have to "pull" all the notes you are playing on the D string cos its suddenly gone a touch flat, that you make enough space for the big guy in front of you to get to the toilet cos its urgent judging by the look on his face! that the guy who actually wrote the song you are singing has just walked into the pub............oh sod it.........just tell the drummer to F*** ***!!! ;-)