I have tried rebooting and it still isn't working.Try a reboot and see if it starts working again.
Have checked that it is enabled in settings?I have tried rebooting and it still isn't working.
Not a known bug: doesn't occur for me, and haven't heard of anyone having the problem.
Try restarting your iPhone by holding in the lock/power button and home button together until the apple logo appears and the phone restarts.
If that doesn't work, I would toggle Siri off and then on again in Settings.
It's an upgrade install from OTA. Should I do an 'Erase All Content and Settings' and then restore from an iCloud backup?In that case, I have no idea. If you haven't been using the beta long, and on a fresh install, I would go ahead and restore the iPhone.
Otherwise, perhaps the issue will resolve itself, or a later beta update will kick it into action.
It's an upgrade install from OTA. Should I do an 'Erase All Content and Settings' and then restore from an iCloud backup?