From the dark recesses of memory.....It must have been late '79/early '80 when I stood in for him since I vaguely recall travelling up from Milton Keynes.....I think Jack must have been seriously ill at the time so that somebody (Keith Kendrick??) quickly organised a group of stand-ins for him.
I've looked at his YouTube vids- a couple of tempting songs there.....Ahhh, that voice......