I am "letting sleeping dogs lie" ..............
I've been out of the UK since 1967, the child is now over 70 and, even if s/he goes searching for the father, I'm not going to see any messages or stories printed in local newspapers, etc. There are few relations left in that area, all of them even older than I am .......... and if one of them did see something, I can bet that it will be the same as happened in the 1980s. We saw one cousin on a visit back home, and she suddenly said "Oh, there was a piece in the local paper about 2 year ago asking if there were any members of the xxxxxxxx family still living here. Of course, I didn't reply."

She was a member of that family via her mother!!
I'm satisfied in my own mind that I will never know anything more than I do now .......... but I am also satisfied with the knowledge that my own child is aware of the fact that she has another cousin "somewhere" so that the child is not completely lost to the family.
I've never solved the problem in my mind as to whether my brother's 2 surviving children should be told that they have/had a half-sibling.