I've been experimenting with the couple of different images I had lying around. Oddly, the initial ones that had working ethernet [disclaimer: no screenshot.. anecdotal evidence ] no longer do. The official ODROID-C2 image from Hardkernel is fine (no surprise there) BUT interestingly, the IP I had delivered via DHCP is different to the one I had before eth0 disappeared. My router's DHCP table shows some... interesting behaviour.
The bottom entry is the Hardkernel image that works. The top one was my functional Armbian image before eth0 decided to PHY off.