First, I try to install it by just copying the installer app - "can't be verified".
Then I make a bootable USB stick using DiskMaker X - "can't be verified". I run an integrity check on the installer - all good.
I then try running `/path/to/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/USB-STICK --applicationpath /path/to/Install\ OS\ X\ El\ Capitan.app --nointeraction` - "can't be verified".
Finally I give up and the system is installed via the App Store. The download fails once, it finally completes, and I am left clueless as to why I couldn't reuse the old installer.
An "expired certificate" error message would have saved me many hours. It's unfortunate for the user that Apple puts so little emphasis on letting them be their own tech support.