I saw Maggie Smith in the Noel Coward play "Private Lives" when she played Amanda to her then-husband Robert Stephens, who played Eliot (originally played by Noel Coward himself and Gertrude Lawrence in the 1930's original production.) This production was at the Phoenix Theatre in London, and, I believe, was directed by Sir John Gielgud. When I was an A.S.M. in Colchester in the early 1970's we did our own production of this play and I have been a Coward fan ever since. His plays never seem to be out of fashion, unlike a lot of other playwrights of that period. Just as amusing as when they were written and in some cases more relevant today than when they were first produced. Such plays of his, as "Hay Fever" and "Blithe Spirit" are constantly being revived.
As for Alan Bennet, I have also enjoyed his work. He did a lot of plays for the BBC and a number with Dame Thora Hurd. He wrote a series of monologues for the likes of Thora Hird, Maggie Smith, and Patricia Routledge and called "Talking Heads." The first play of his I remember was a black-and-white television film about a gang of Edwardian cyclists. Very little plot and, being set in that idyllic realm immediately before the First World War, had a very poignant ending. A definite 'must see' if you get a chance. He's since done most of his writing work for the National Theatre, such as 'The History Boys' which was made into a film in the early 2000's.