FWIW Will Fly's session where the emphasis is on improvisation around the tune (trad or otherwise) sounds excellent - if a bit of singing is OK too!

Yes, there's singing as well - it's an eclectic session, and what is played or sung depends on who turns up. We just sit round a table or two and go round the table, allowing each person - if they want to - to play or sing whatever they choose. People join in if they know it, or if it sounds appropriate to join in. If you don't know the tune, you're encouraged to have a go. If I'm singing a song, for example, and others are playing along on their instruments, I invite people to play a chorus - improvise whenever they want to - sing a harmony - essentially make music. There are several traditional tunes sessions in my Sussex area - and very good ones {"English tunes and no bloody singing!"} for those who want exclusively that kind of session.

The monthly session (last Sunday of the month) at the Bull in Ditchling is the same kind of session as mine - though with many more musicians attending - and it's the one on which I modelled my own session: come as you are, do what you want, as long it's acoustic. As it happens, we usually get a nice mix of traditional tunes and songs, plus some American old-time and early country stuff, jug band music, even the odd Richard Thompson ditty - just to cheer us all up...