A Bunch of Mistletoe by Lynne Connolly

Companions aren’t supposed to marry dukes.
When Matilda, chaperone and aunt to the most scandalous family in London, takes a tumble from a tree, she’s rescued by the most unlikely saviour. Toplofty stickler Harold, the Duke of Trensom, is more than annoyed to find Matilda on his property.
Worse when he finds himself falling for her.