The Spring Promise by Sara Bennett

The first novella in the Starling Hall series, set in the English village of Barton Lacey.
Will Starling and Molly Lacey have been best friends all their lives. Molly always believed they would marry, until Will set his sights on a pretty newcomer. Desperate to marry the flirtatious Celeste, he asks for Molly’s help. When she demurs he makes her a promise.
She can have anything she wants from him.
All Molly wants is Will, but it seems he is lost to her. She agrees to distract Will’s rival Mark Hunter. And then things begin to go very wrong.
Can they realise in time that their friendship has turned to love?