At the risk of name dropping, McColl and Seeger gave me the full story when I interviewed them. I asked about the opportunity the royalties gave for them to further their trade without having to worry about putting meat on the table.
That supplementary question says it all. If there were claims to the song they would have been made.
The combination of chart topping hit in 1972 by Roberta Flack and covers by Frank Sinatra, Elvis Presley, Johnny Cash, London Symphony Orchestra, Stereophonics etc.
The song netted them millions of pounds. Worth a legal punt if you had a claim?