The Arrowhead Moor adventure / Fleur Hitchcock.

Summer is sweet for cousins Lucille, Chloe, Aiden and Sol, staying with Grandma and Grandad means they can cycle and sail all around the coast without any grownups in the way. But out in their boat Chloe sees a terrified face at a window and they realise there's something rotten about nearby Applestone Hall. Could the face belong to recently kidnapped young George, the only child of a rich local businessman? The cousins soon find that no one else will help them uncover the truth. Certain that the villains are on the loose in the countryside, they must use all their skills and cunning to conduct a daring rescue and capture some desperate criminals.