I’ve effectively started with a new iMac following my disastrous OS Sierra upgrade that resulted in the Macintosh HD being readable but not writable - and not bootable! As the OS install partition was still OK, I managed to reinstall El Capitan on an external USB drive and then did a restore from Time Machine to the USB drive.
Having now got a backup to my backup, I reformatted the internal drive, installed Sierra successfully and then restored from Time Machine. No major issues and the 2010 iMac seems to be running faster so I’m content.