Martin Carthy in the notes to "Palaces of Gold" on "Crown of Horn" explicitely names the Aberfan disaster as a reason for Rosselson writing this song. But bjgj is correct in pointing out that this is not a song about that specific disaster. The song lines closest in content to the Aberfan disaster go like this:

"...that if you don't want to be buried alive by slagheaps [the Aberfan event],
pitfalls and damp walls and rat traps and dead streets,
arrange to be democratically born
the son of a company director..."

Wolfgang