Hi Chris,
I have sorted it out. It was really quite silly; I’m using Roots theme which changes the ‘uploads’ folder (not the default /wp-content/uploads), so I just needed to place the mappress/icons folder (with my custom icons) in the actual uploads folder.
Although it is pretty obvious, maybe you should consider rewording your documentation from
If you have custom icons, you must move them from the
/icons/directory (under the MapPress installation directory) to/wp-content/uploads/mappress/icons.
to something like
If you have custom icons, you must move them from the
/icons/directory (under the MapPress installation directory) to/your-uploads-folder/mappress/icons(default is/wp-content/uploads/mappress/icons).
Thanks for your interest!