If, as suggested above, this is a unique song in that it describes the working conditions of canal labourers - and I think this is correct - where did it first resurface?

It was recorded on three lp's in 1975/6 :
Straight from the Tunnel's Mouth - The Boatmen
The English Canals - John Kirkpatrick, Sue Harris and Jon Raven
The Bold Navigators - Jon Raven, John Kirkpatrick et al

It looks as though Jon Raven was the first to get his hands on it but from what source?