BEIJING—Cruising around Beijing in a silver Jaguar with "007" in the license plate, Neil Heywood seemed to relish the air of intrigue that surrounded him.

In meetings, the British consultant hinted about his connections to Bo Xilai—the onetime Communist Party highflier—but often he would refuse to hand over a business card. He spoke Mandarin, smoked heavily and worked part time for a dealer of Aston Martin cars, the British brand driven by James Bond. Some thought him a fantasist, others a fraud.

But...