The Guyanese migrant experience of 1970s London gets the big-stage treatment Plays about the Windrush Generation are no longer a rarity, but it's still unusual for revivals of black British classics to get the full resources of the National Theatre. Guyana-born playwright Michael Abbensetts, who died in 2016, is often mentioned in books about black British drama, but his plays are infrequently revived. So it's great…