Hello adelaideviewer,
I'd suggest first trying auto-detecting the pedal again, making sure that, when prompted, you move the physical expression pedal back and forth a few times across its full physical compass. During auto-detection, Hauptwerk listens for the highest and lowest position (MIDI controller) values that the pedal transmits, and maps that range automatically to the range of virtual pedal. Hence it's important that you move the physical pedal as far as it will go each way.
If that doesn't solve it, when Hauptwerk isn't running, try using something like MIDI-OX ( http://www.midiox.com/ ) if using Windows, or Snoize MIDI Monitor ( https://www.snoize.com/MIDIMonitor/ ) if using a Mac, to view the MIDI mesages the pedal is sending. You will then be able to see whether it's sending a good range of controller values smotthly (and broadly linearly) across its full physical compass. (Ideally it would send controller values ranging from 0 to 127, but many pedals send a somewhat smaller range, which isn't a problem unless it's very small).