Turns out that VirtualBox has had a “bug” for around three years where if “sendfile” is enabled in the web server you are running it won’t serve a file after it has been changed on the file system mounted from the host machine.
This function, a kernel call to copy files directly from disc to TCP, is used by both NGINX & Apache to help serve static content more efficiently.
The issue is documented here ….
Also, it was a good lesson in Google early (hence the Picard faceplam) !!!