I was just about convinced that my memory had totally invented "Jazzbo's Holiday". Unfortunately my memory is not creative, merely incompetent. I did a quick search and came across this site Famous Jug Band
It seems that " Jazzbo's Holiday" a banjo/guitar duet was recorded by Clive Palmer and Robin Williamson on an LP "Edinburgh Folk Festival Vol. 1". (I wonder where my copy of that album went.) My memory, useless but plausible, shifted this track onto the first ISB album. Sorry about that.