Thanks for the response. It was nothing to do with the VM SuperHub.
I've got to the bottom of it but haven't quite fixed it yet.
The problem was connected to the software firewalls on the PCs. I run McAfee Total Security on both. It is their firewall that is blocking access to file and print sharing. Despite allowing NETBIOS access through their firewall I still have problems.
If I turn the firewalls off then everything is OK.
If I use the Windows firewall and allow File & Print Sharing then it works. So I just need to sort out the McAfee settings. (I don't like using the Windows firewall as McAfee insists that my machines are not fully protected!)
The question as to whether I need software firewalls at all is another matter.