The Jillions are sent to stay with an old lady in a large, run-down country house.  Mandy is at her bossiest as the young entrepreneurs try to help by opening the house to the public (and treasure-hunting villains).

Lutterworth Press (1953)
Armada (1963)
Armada (1967)
White Lion (1975)

Criterion Books, American edition (1967) Note the change of title