The car was built by Adams & Robinson initially, but work progressed at a snail's pace and the car was eventually purchased, unfinished, by John Willment, who had the work completed and then used the car for a couple of years (I seem to remember the car took five years from purchase to completion!).
I can't remember the name of the client who originally ordered the car off the top of my head, I can check later. Although often known as the Willment Jupiter, this is slightly disingenuous I feel as the vast bulk of the work and the design was by Adams & Robinson. Perhaps the topic title should be 'Willment' Jupiter by Adams & Robinson.