The American dramatist Lorraine Hansberry (1930-65) is known only for A Raisin in the Sun, her 1957 African-American domestic drama that has just completed a British tour by the Eclipse Theatre Company. That play – and the song “To Be Young, Gifted and Black”, written as a musical eulogy by Nina Simone, a friend and admirer – seemed likely to be her cultural legacy.

A major reason for Hansberry’s small output is that she died so young, but, during her final days in a cancer ward, she was collating and amending drafts of an uncompleted play called Les Blancs which, after work by several dramaturgical hands, now receives a National Theatre production from Yaël Farber, the South African director whose radical versions of Strindberg’s Miss Julie (as an apartheid-based “Mies Julie”) and Arthur Miller’s The Crucible have recently had much-admired runs in Britain.

It is risky to describe what we are seeing as Hansberry’s play because the text was constructed in the late 1960s by her widower, Robert Nemiroff, and now Farber – with another dramaturge, Drew Lichtenberg, and “script consultant” Joi Gresham, who runs the Hansberry estate – has refashioned the script from all the extant drafts. The result is an epic drama of almost three hours that feels like a collaboration between James Baldwin, literary spokesman for the rise of black political power, and Graham Greene, the poet of spiritual and political tension in foreign lands.