The new bundle is stored in /User/Library/Carrier Bundles/iPhone. Edit the carrier.plist in the bundle there and Reset Network Settings. You can also edit the CellBroadcast item to change the new broadcast notification settings.
EDIT: Remove the files under /User/Library/Carrier Bundles/Overlay before resetting.
Thank you this worked perfectly. I did have to reboot twice before resetting the network settings, just as before to rebuild the overlay.
Hopefully this works for others too.