Apparently, he adapted the tune from his memory of a local singer that he heard as a lad. Local doesn't mean, of course, a source singer, but could have been the local C&W wannabe.

Da da de da de da da plonk plonk plonk plonk
da da de da de da plink plink plink plink

Nope, that ain't trad, trad would have more das and des and no plinks or plonks.

So, local singer, local legend, yep that's trad innit? One could lose out on royalties that way.