The Lord of the Manor

This is a mystery novel that revolves around the Castleraynes. The Castleraynes were once powerful in Yorkshire, but their wealth and estate gradually dwindles under three generations of squanderers. Gertrude Castlerayne is charged with running the household. However, she is having difficulty making ends meet. A proposal is sent to the Castleraynes. The family is asked to allow hotel guests to picnic and wander on their land. And they would be compensated handsomely if they agreed to it. Clifford, a mysterious stranger from London, comes calling at that very moment the proposal is received by the Castleraynes. Who is this mysterious stranger? What connection does he have with the Castleraynes and the proposal?