I found a copy of the lyrics here and it has this additional verse after the "Granddad" verse:

And then your little lad, all innocent, says, "Dad,
Can ten-year olds get in the family way?"
You smile and say, "Not yet", but your legs turn sort of wet
When you hear him tell the girl next door, "I said we'd be OK."

Also, here's a discography on Fred Wedlock's web site which says JUST KIDDING is on an album called "Live (with Chris Newman)".