Input Echo must be on else it wouldn't work when Record is on. There is a setting in Preferences to allow Input Monitoring during playback, which is newish (so probably in CbB but not Platinum), which is likely unticked.
Ta. I fudged it in the end (app on another machine to extract the installer - evil thing which leaves a service installed and running after uninstalling it!), so this would have been much cleaner 🙂