Thank you for your help and explanation! I found the reset procedure. Upgrade worked like a charm.John used to monitor the changes that RMerlin was doing to his firmware (beyond even 380.70) and if appropriate apply them to his firmware. So as well as package updates you also got bug fixes, security updates, etc. He did not change the base GPL or add new features as that is against the LTS principle of this firmware. So this is a patched 374 firmware and not a 380 firmware.
Here is the reset flow if anyone reads later:
1. Powered down the router
2. Press and hold down the reset button and powered up the router
3. Power led starts to flash slowly and let go of the reset button
4. Power led keeps flashing slowly
5. Connected LAN cable to Lan port 1
6. Set PC IP details to :
192.168.1.10 static ip
255.255.255.0
192.168.1.1 gateway