Thanks PH ...
I had previously tried using the IP address of the server instead of the hostname. And having tried it again, it still fails.
I don't know why Apache2 collapses with a SIGTERM when I am certain the the configuration is correct. And even if it wasn't I'd have thought that any error should be trapped.
It's a bit disappointing, but I am going to re-install the OS and Apache2 in a minimal server so that re-installation of the whole thing during the discovery of why this isn't working is easier. If I can then get a set of configuration files that work (!) I will try them with a fully installed fully configured server.
You may not hear from me for a while but thanks for your patience!