Custom firmware version

A full list of all versions is on the Wiki Here (Click) - it all depends what version of the standard firmware you've got as the CF is written to overlay on top of a standard.
It's up to you. The latest version (if you have the beta version of the Humax firmware v1.02.27) is v2.1. If you have the latest official Humax firmware of 1.02.20, then custom f/w 2.00.

edit: oijonesey pipped me too it!
I can't seem to find out how to upgrade my custom version, I know it will be on here somewhere just can't find it.