The White Stag Adventure Paperback by Rennie McOwan (The Clan Series)

There were dark stains in the lay-by where the car had been. To Gavin they looked like blood!
Who were the men in the car and what was their interest in the great white stag which would appear mysteriously on the hills.
Gavin and his friends Mot and Michael Stewart under the resourceful leader ship of their sister Clare are planning to spend their summer holiday turning a ruined castle into a den for themselves. Their activities interrupted when they are caught up in a series of mysterious and frightening incidents…

The Clan series of four fast paced children's adventure novels follows the adventures of Gavin and his intrepid friends the Stewart children, Clare, Michael and Mot.