Version one, check2xsf, is no longer advertised. Versions prior to 1.14 have bugs when parsing wavefunctions from parallel Castep calculations, or versions of Castep from 2016 and beyond: they are not recommended.
Version two is available either in the form of binaries, or source code under the GPL version 3, as below.
Linux, 2.05b, with spglib: Linux.
MacOS, 2.05b, with spglib: MacOS.
Man page, nroff source: man page.
Man page, html formatted: HTML-formatted man page.
c2x_2.05.tgz (A C compiler is the main build requirement).
Test suite: c2x_test.tgz.
Ideas, and source code, contributions are welcome! But, if contributing source code, you must agree to assign all intellectual property rights to MJ Rutter, the main author of c2x, so that it is possible to release c2x under other licences in the future. I do not believe that it will be possible for me to make my fortune by commercialising c2x(!), but I do not wish to be unable to contribute its code to a project which is not GPL v3 compatible.
See also the c2x futures page for features under development but not yet in the public release.
Contributions etc. via email to mjr19 @ cam.ac.uk.