Jet Set Willy: The Final Frontier was really Jet Set Willy 2. The original for Spectrum had a much smaller mansion of 60 rooms, but when ported to Amstrad CPC the designers (Derrick P Rowson and Steve Wetheril) decided to expand the game, more than doubling the number of rooms to 132. Later, the expanded version was ported back to Spectrum, and from then on was considered "Jet Set Willy 2". Also, there were other versions, at least three, that featured completely new maps and rooms, but apparently were only released for the Spectrum.
A PC version of the first Jet Set Willy, by Andy Noble, is also available.