Whois says nada about who owns "a website".
A website consists of at least two parts:
1) The technical framework running it.
2) The content put into the website (typcally content in databases and xml files).
assume you created the 1) part of it, and if you did not receive wage, it is your "intellectual property". In theory at least, you should therefore be able to have a solicitor send a claim on either them paying a fair amount for the "CMS", or that they within a fair timeframe stop using you CMS and migrate it to another system.
I don't see that you not having a company behind you should deby you your intellectual rights.