However, Now On Tap is missing from my system. Holding down on the home button just brings me to the Google Now cards screen. This happens on the Google Experience Launcher, the 3rd party Nova Launcher, and various apps including Facebook, Inbox, Telegram and WhatsApp.
In case I accidentally disabled it during setup, I've looked through the settings to try to re-enable it but haven't been able to find the option.
As far as I know, Cataclysm doesn't disable Now on Tap or affect it in any way. I previously flashed the stock system image for 6.0 where Now On Tap was present.
Just in case, I checked: the Google apk version is 126.96.36.199.arm, which was updated on 22 Oct 2015 so that's probably not the issue.