Tynan, a very great supporter of Welles during much of the 1950's, who saw him as breath of creativity in the smoky London air, may have been trying to adopt American slang, chic at the time, not realizing that his essentially anti-racist American counterparts would not have used the epithet, which was a crude Southern regionalism.
In his use of the N-word, a second factor came into play. The British, at least many of them in my experience, because of their Colonial underpinnings, perhaps, used the word in everday life for the color black.
This usage was certainly racist, but in many cases innocent of bigotry.
I remember that, in 1955, the U.S.Army Anti-Aircraft in Britain, as a gesture of good will (and a deal somewhere?) insisted that the troops use for footlocker inspections Tuxsan(sp) shoe polish, which had an artillery red can. Tuxsan's black polish was named "N*gger."
Two black Military Policemen on TDY came into the Lakenheath Base PX, asked for shoe polish, were shown the Tuxsan display, and they went berserk. The two were courtmarshalled , as I remember it, but just before, I came home, the regulation was changed.
In sensitivity to language, at least, we've come a long way.