I just played through the music and it is very similar to what is sung, enough so that there's no doubt in my mind that it's based on it.

I studied music at Bath Spa University a few years ago and one of the professors was very friendly with Gavin Bryars. I was told at that time that no-one had ever found the origins. I've just emailed another professor at the university to tell him about this, in case they didn't know.