"What I cannot fathom is how, if he plays through the tune (sort of) on a plastic whatever it is, he can then start singing a verse in a totally unrelated   key."

It's a rare skill, normally requiring years of rigorous training. In his case a natural gift. Most of us couldn't hope to attain that level of musicality; we can but gaze in wonderment and humbly bow to His musical superiority, boundless knowledge and indisputable wisdom.

If them Watersons would only take a leaf out of WaV's book, they could be quite well-known by now.