Well Done Secret Seven by Enid Blyton
 
Because the Society shed is too hot for their meetings, the Seven built a meeting place in a large oak tree in Windy woods. They tie large planks to the branches to sit on and bring cushions and find a nice cubby-hole in the tree to store their food. But they discover someone else is hiding in their tree - because of something he over-heard which had caused him to run away. He tells the Secret Seven what he remembers. A red pillow - MKX - a date, Thursday 25th, and Emma Lane. Will the Secret Seven be able to find out what all this means?
 
This is the third adventure of the Secret Seven, Illustrated by George Brook