It seems to work if I put two markers close together. I think the problem is to do with calculating the extent of the map required to show markers / track etc. When the only feature is a single marker the extent is a single point. Still this doesn’t explain why it works for grid references.
I don’t have time at the moment to work out how WordPress plugins work and get up to speed with the Javascript, and anyway it may be a problem with the OpenSpace API.
The problem will go away when I have more data to display.