The "Green Room" at the Troubador takes a fair bit of beating. An old coal hole (Coil oil ?) behind the stage, with thick glass tiles in the roof, that were set into the pavement above. That's where I first met The Watersons ;)
There was a club in the barn of a farm, where we all sat on hay bales, Oxfordshire, near Turville Heath I think, it was a very TV studio idea of a folk club setting. Can't remember the name at present, but I know me old mate Leadfingers used to there. It'll come back to me eventually.