It's well known that Bert didn't make the song up. He adapted it slightly from a Lake District miner's song, but changed the location from Borrowdale graphite mines to the better- known coal mines of the North East, which is why subsequent researchers didn't find anyone who sang it. It was of course originally The Blacklead Miner.

Don't work in the pencil mine,
Across the page they draw a line
To make sure that the point is fine
Or they fine the blacklead miner!

Borrowdale is a terrible place,
They all pack pencils in a case
With filthy hands and blackened face
Those dirty blacklead miners!