No I didn’t forget to change my gametype, however I have just solved my problem. I’ve gone back to the interface and it looks like I already had all the code I needed, I just hadn’t joined it up into one system.
It looks like I still needed to specify my HUD as a target for the message for the interface, despite my interface already being referenced in the HUD and the class blueprints. Basically, in the class blueprint I have:
My Interface blueprint is a basic interface function with no variables in it, and in my HUD I have an event for the interface function that now fires properly.
Thanks for your help alperenakyuz, I really appreciate it! 
As an observation to the developers:
I did try finding a node to represent my HUD by dragging off the target pin of the ‘interface message’ node. This however didn’t show anything. I COULD find it by dragging off the ‘get player controller’ node and then connecting it to the interface message, both of which give me the same result, so why can’t I see the ‘get HUD’ node when I drag off the message target pin?