Hardware/software list on Mac, showed that Ethernet 'AppleBCM5701Ethernet.kext' file had had been disabled.
Fix is contained in this forum link;
I've downloaded and applied the file contained in the following link;
I rebooted my iMac and the Ethernet port has recovered.
Please note the stanadrd PRAM NVRAM reset will not fix this issue.
Hope not too many other people have been caught out by this poorly managed update.