Old GCHQ building in Cheltenham demolition begins Demolition work has started on an iconic building on the old GCHQ site at Oakley in Cheltenham. M block, which became known as the "Green Monster," is being pulled down to make way for more than 300 new homes. Much of the building, which was built into the side of a hill, was reinforced to survive possible attacks and had meeting rooms underground. The site, where GCHQ started life in Cheltenham in the 1950s, closed down three years ago.