The Duke of Marlborough offered his granddaughter Harriet with an exceptionally large dowry .Why?

I guess you are referring to the "first" Duke of Malborough, if so, I thought that Harriet was the eldest of his daughters ? He had two sons (who died) and three daughters.

Harriet would then have the honours of being next on the list according to the letters patent . Lacking any males, the Act amended the letters patent so that, failing the heirs male of his body, the titles and honours would pass to Lady Harriet, his eldest daughter.

Indeed, Mozart. Here 'the Duke of Marlborough' would always mean the famous Duke of Marlborough, who happens to be the first Duke: we don't use the numbers unless there's a risk of confusion between the current Duke, Earl or whatever and the historic bearer of the title or, of course, a pub called the same. ('The Duke of Westminster' always means the current one because he has no famous predecessor He's the sixth Duke but surely nobody, apart from Gerald Grosvenor himself, has ever heard of the others)

As to Harriet being a granddaughter, that puzzled me too. If she's a granddaughter what's happened to her father, the man who would be offering a dowry? And I can't find any granddaughter, Harriet, of the great man , on the family tree.

'Granddaughter' comes from the Daily Mail's review of the book 'The Kit Kat Club' by Ophelia Field, which gives the story.The word may be a typo in the review

Anyway it doesn't spoil the story if Harriet, the daughter is intended.Let's read it as her case.It's an amusing , if British, tale.

Have a go!