Starting off with a working REST interface, I can access the CSP Application definition screen for the application (System > Security Management > Web Applications > Edit Web Application), and "turn off" the application by clearing the Enabled: [ x ] Application checkbox. From then on my REST calls return an HTTP 404 error. (That's expected).
However - one expects that re-enabling the Application in the CSP Application screen, would return my REST services to a working state, however, this is not happening. All REST requests continue to return an HTTP 404 error.
SOLVED: I have solved this clearing the CSP Gateway cache, that is, by accessing the CSP Gateway Management pages, selected System Status, then, located the REST URLs from the Cached Forms column, and clearing this row. Once cleared, my REST services work fine again.
(I noticed that the URL appears as a cached item, only once the CSP application has been disabled and not whilst the REST services are functioning).
Although I have my solution, I'm wondering if this is expected behaviour.