Hi Brian, just had to check myself. Found the book online and it's definitely a single cylinder, some text from the book below.
"For research purposes he had adapted the basic castings of his Hornet engine to make a new four-stroke 7-cc. engine running on petrol with a little carburetor, and in place of the reduction gear he had fitted a governor; ignition was by a tiny magneto of his own design and a miniature sparking plug. The whole lot mounted on a little baseplate was about four inches long, two inches wide, and two and a half inches high. It was an easy starter. He could flick it into life by swinging the flywheel at one end with his thumbnail, and as it speeded up to the governed revolutions a pea bulb at the other end glowed with the electricity it generated. It had always been a great success in workshops, and he put it in a little box and slipped it into his pocket. "
"the 5-cc. Hornet single cylinder compression ignition engine with its built-in reduction gear"