I take it I still need to keep the existing collider I have on the key though?
Whew… Well, it’s taken me a while, but the system works! 
The door opens once I have the key! Thank you, and everyone else, for all your help! I still need to look further into these TMaps as it’s completely new to me. And they’re not touched on in the course I’ve been following. So hopefully it’s something I’ll make further use of. 