The Secrets of Greystone House. Marcus Attwater

When he was eleven years old, Jack Riley spent two golden months at Greystone House with Gwendolyn, Ian, Rory and Jamie. Seventeen years later his job as a newspaper reporter brings him back, to learn that it wasn't just the children who kept secrets that summer. Those days just before the war were not as idyllic as they seemed, and instead of writing about the discovery of an Iron Age treasure hoard, Riley finds himself reporting on a murder inquiry to which his own memories may hold the key.