Been trying out various things, none of which made any difference:
Restarting machine
Reinstalling firefly
Removing drive mappings to network shares
Using IP address instead of NAS share name
I am now almost certain that it is being connected to the VPN when Firefly scans after machine start that makes the difference. Once the scan has started I can disconnect the VPN at it carries on and will rescan happily with or without VPN until the machine is shut down.
I have VMWare workstation installed and although this is not in use I am aware that installs its own network adaptors so I wonder if this might be a factor. When I get some more time I will try uninstalling VMWare and see if that works, but as it looks right now I don’t think Firefly is going to fly for me 😥