Thank you all for your replies. They all confirm what I already knew but, sadly, didn’t explain why it wasn’t working for me!
I tried using the Session Storage Manager just to satisfy my conviction that server and browser sessions are different.
However, I have found what the issue was and it was my own misguided attempt to test it before actually doing it and it’s flagged up something which is worth sharing.
Scenario: You have an API script which sets a session variable. That var can be accessed by any other API script BUT NOT if you run that API directly unless the session was created by running it directly.
I was foolishly trying the confirm the value in the session var by creating a simple API script and running it directly but it wasn’t updating. Had I not tried to test it this way, I would have just continued to build the API script and it would have worked perfectly. As it now does!
So it’s not just a case of client-side sessions and server-side session being completely different, it’s also a case of how you run the API script.
Thanks everyone. Some really helpful responses. I was typing a different reply explaining what I was trying to do when the penny suddenly dropped so I built my functionality and viola!