The permutations of musical cadences that are easy on the ear are less than the mathematical number per se. Once you start a tune, and the starting line being similar to another, the rest is a forgone conclusion.

However, in this case, McColl's tune for Sweet Thames was very much a nod to the tune used for Recruited Collier. I never knew the man in the way Jim did but I did interview him on several occasions and he spoke of the wealth of tunes lying there for Peggy to get his words around. I am sure I am one of many who sing the final verse of Sweet Thames to just about the same tune exactly, as "going up" at the end of the first line denotes finality. (Hymns provide many examples of the style.)

When I was a teenager, I wrote a song for a rock band I was in, and years later heard a song to about the same tune on about the same subject. Be buggered if I know whether it was coincidence, one if us had once heard the other and forgot the source or, as I said, there are logical conclusions in play regarding songwriting.