Yay! I can log into the VMs but realise there is a problem with DNS. I can ping outside hosts and all seems fine with the network. Started changing network settings in one of the guest OS and no, nothing is happening
After some investigation, this appears to be due to a change in how Ubuntu is doing DNS and that the Guest OS are unable to handle that natively.
Enable two parameters as the user you run your VM's under
VBoxManage modifyvm "Centos 7" --natdnsproxy1 on
VBoxManage modifyvm "Centos 7" --natdnshostresolver1 on
VBoxManage -natnetwork list
Make sure all VM are stopped and then run this, it willl crash VirtualBox but makes sure next time you start VirtualBox the new network settings will apply
VBoxManage natnetwork stop --netname "NatNetwork"