OK so having downloaded El Capitan
onto another Mac (running Monterey) I then double clicked the Install (InstallMacOSX.dmg) file and then double clicked the PKG file within it and installed the El Capitan installer onto my Macbook (which does not install the OS, just the installer)
I then checked that the El Capitan Installer app was in the \Applications folder on the Macbook and it was.
I then changed the name of my 8GB USB stick (plugged into the Macbook) to 'MyVolume' and ran the following command from the Apple site
instructing how to make a bootable USB:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
Which then wrote the USB stick successfully although it did take quite a long time. I safely ejected the USB from OSX first, and then removed it physically. Then I plugged it into the old iMac.
I then restarted my iMac with the option key held as soon as the chime sounded. I was given the choice between the Mac disk and the El Capitan installer, I chose the latter.
The USB disk was signalling activity and the progress bar with the Apple sign was displayed. This again took a long time. The progress bar the restarted and I was presented with the dreaded Mac Utilities screen again, but this time the Install OSX
option had a slightly different icon.
I chose this option and was presented with the OS X El Capitan Installer.
I proceeded to install OSX El Capitan to the Mac Disk which appeared to work as hoped. I could now see the full graphical OS X El Capitan installer screen with a progress bar at the bottom.
I will post back what happens.