@Dan Cronin
If the base station does not show up in the AirPort Utility, give the following a try:
# Temporarily connect your Mac or PC directly to the Express by Ethernet, and then, try using the AirPort Utility again.
# Try using an iPhone or iPad instead. These tend to work “better” in finding a base station. Just use the AirPort Utility app.
# Make sure that your Mac or PC is configured for a minimum of IPv6 Link-local only. The AirPort Utility relies on both Bonjour and IPv6 to discover a base station.
# Try a “factory default” reset on the base station.