Claire Foy born 1984, Paul Bettany born 1971.
Ian Campbell, 11th Duke of Argyll, born 1903, married Margaret Whigham, born 1912.
So the age gap is not too different.
Married 1951, divorced 1963. So he was 48 when they married and she was 39. He was 60 at the time of the divorce, she was 51.
That's very different, as you say, @Piggywaspushed. Claire Foy isn't 39 yet, so far younger than the Duchess at the time of the divorce.
There must be some charismatic and beautiful actors born in the early 70s who'd have been more realistic choices. Paul Bettany looks older than his age in this so not bothered about that casting.
Helena Bonham Carter is probably a touch too old now.