Mulbridge Manor by James Reeves
 
Strange old Miss Matilda Jay asks Richard Masters to help her find her brother's will. Without it, her unscrupulous nephew will inherit the Manor.
Richard musters his friends and they organise Headquarters in an old barn.
The search for the will leads first from the library of the Manor to an old book-shop. Then comes the clash with two shady gentlemen known as Seedy and Dusty. The children's moonlight feast in the barn is mysteriously interrupted and a new suspect comes on the scene.
 
Even when the will is eventually discovered the suspense and excitement are by no means over, for Seedy and Dusty steal it. Then, just when it looks as if the children have failed, events take a surprise turn.
 
There is a light-heartedness about this adventure which is part of the summer world of childhood; plans are laid at a picnic tea on the Heath: the barn by moonlight takes on mysterious new possibilities; the climax is reached in a green lane, full of bird song. The reader becomes part of the story from the first page.