Do I even need to log in to the console, or should I just leave it for a few minutes at the console prompt and then try to connect via a web browser (assuming it gets the right address).
First the analysis what went wrong: during system init Network-manager is stopted and network.service takes over. In your case network.service was not able to fetch a ip from the dhcp server. system-init gave it the most common ip out there: 192.168.1.1
A hack to salvage your install: nano /etc/sysconfig/network-scripts/ifcfg-eth0
And give it a fixed IP and network properties matching your situation:
here is mine:
cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=none
GATEWAY=10.0.10.100
IPADDR=10.0.10.5
NETMASK=255.255.255.0
NM_CONTROLLED=no
NSLABEL=
ONBOOT=yes
TYPE=Ethernet
USERCTL=no
then restart network: systemctl restart network
check your ip : ip a
Go to the web interface (on his IP) and assign your fixed ip again there.
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It all seems to be going OK now - many thanks for all your help.
Is there a better place to put all the raspberry pi nethserver questions?
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Glad you are sorted out ! and as said the feedback is valuable 
No it’s fine here, you may give the extra tag arm:
This triggers the arm team to have an look, and others know it may be arm specific.
Could you mark the write-up form my (poor) memory as the solution of the topic, after all it did get the RPI4 booting from the USB dive 
I meant mark it a solution here:
So other people can find a answer quickly without reading the hole post. 