At the beginning of "The Destructors," the August bank holiday is mentioned as the time when Trevor or "T" joins the gang. The August bank holiday in London is at the end of August, that places the setting in late summer. It is some years after the Blitz, so it is approximately the 1950s. The gang lives in Wormsley, a fictionalized area that could be a section of London, Buckinghamsire, Essex, or Hertfordshire, since the Wormsley Common Underground Station is mentioned, and the subway runs in the aforementioned locations.
The area in which Old Misery and the boys live must have been a nice neighborhood at one time, given that the house in which Old Misery lives was designed by the famed British architect Sir Christopher Wren. The area, however, is no longer upscale, as evidenced by the working-class speech of the boys and the proximity of Old Misery's house to a parking lot. Moreover, many of the neighboring houses have been completely destroyed or heavily damaged by The Blitz.