The car described in the book is build using parts from a Mk1 or Mk2 Ford Escort with front calipers from Ford Cortina, but people building the car have used parts from many different cars to make their version of the Locost.
Both Colin Chapman and Ron Champion have a background in the 750 Motor Club and the design of the Locost is based on a Clubman's Race Car designed and built by Ron Champion in 1963. The rear is ofcourse "inspired by" Lotus 7.
Many different companies make and sell parts and complete kits for building the car, some examples are MK Engineering (who have continued on the concept and now offer their MK Indy, based on the Ford Sierra), RaceTech with their Lada based ESTfield and many more.