The Guardian by Carole Mortimer

Hunter St. John, the Duke of Lincoln, is far too busy hunting the person who murdered one of his closest friends to deal with the problem of his young ward, Miss Evelyn Gardener. Indeed, for the most part he chooses to forget her very existence. That is made easier by the fact he sent her, along with her governess, to his remotest ducal estate in Yorkshire years ago and has seen very little of her since.
Receiving a missive from that governess, informing him that his young ward has run away, is not only inconvenient to his own plans but disobedient in the extreme on hers.
Something Miss Evelyn Gardener will soon learn to her detriment.
Once Hunter has succeeded in finding her, of course…
Evie is tired of living in the country and being ignored by her guardian, the toplofty Duke of Lincoln. To that end she has decided to travel to London and make him acknowledge that, at nineteen years of age, she is no longer a child for him to ignore.
It is hardly her fault if, on the long journey to England’s capital, she is waylaid and captured by a group of footpads.
Nor, when she hears those men discussing demanding a ransom from her guardian in exchange for her release, is she at all sure the Duke of Lincoln will bother himself to pay that ransom. Indeed, she believes the duke might happily wash his hands of her.

When the two of them do finally meet again sparks certainly do fly, but not of the kind either of them had expected…