@malwaretech
The latter is only true for detention under section 2 of the MH Act(s); you can't automatically be released under a section 3; furthermore there are sections related to criminal acts where release is at the *sole* prerogative of the Home Secretary (mostly those referred to as "37/41" detentions).
He might well end up being detained for life, but as we cont know the section being used related to his sentence, we don't know.