Add listing page doesn't show excerpt – short description

@Mark, thanks for the info, I’ll try incorporating in a child theme at some point. You are right, anyone who doesn’t know what they are doing would be better using the plugin and the plugin also has some configuration options and extra functionality which will be useful for many people.

My own requirements were very simple and my small mod (adding a few lines) achieves exactly what I want so I’ll stick with it and avoid introducing an additional plugin to my system.
Part of my motivation was also to show that to include this as part of the standard theme appears to be very simple. I’m happy to share it if anyone want to use it or test/critique it.