Chosen Solution

I have been able to complete this task with no problems on other Mac minis of the same model, but I am perplexed by this. When removing the top left T6 screw that secures the fan and heat sink cowling to the logic - on left in this picture: https://d3nevzfk7ii3be.cloudfront.net/ig… - I was unable to lift the fan on its own until I disconnected the cowling from the logic board so that I could lift them both together.

As I lifted them clear, a small spring popped out. I am sure that I could replace the pair of them still coupled, but can anyone tell me how to separate them please? Is this a pair of screws (one screwed into the other)?

Yes, I think the top screw is screwed into the insert screw that goes to the frame. Grab the bottom screw and turn the top screw until it comes out.