oh dear, I'm afraid Rolf Harris recorded this. I believe it was Five Young Apprentices.

Five young apprentices, working on the site
Nobody knows, nobody knows.
Five young apprentices trying to get it right.
No-no-nobody knows.

I can only remember fragments:-

"Don't stand so near the edge my lad,
We're on the seventh floor!" Then there were four.

And the last one was something about "Don't strike a match near that dodgy gaspipe son!" (Then there was one.)

The chorus was:-
And nobody knows where the other one went... etc

Youtube has blocked off some of his songs (for obvious reasons)