Calling Function via REST API with API Key gives 401 using Zoho Developer

Calling Function via REST API with API Key gives 401 using Zoho Developer

Hi,

I created a couple of functions using the one month trial of Enterprise edition, which I was able to call using the API Key method from Postman and from an external site.
Now that my trial has expired, I have created the same functions in the Developer edition (https://developers.zoho.eu/) and can call them from within Zoho Function editor, but when I attempt a call via REST API with API Key from Postman or my external site script I now get a 401 Unauthorised response.

Is external calling not available in the Developer edition, perhaps?
Many thanks!