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Thanks for your response :-) i have now fixed the box so it works perfectly. The problem i think was because i scaled the box up to make it much larger, this obviously made all the tolerance gaps larger as well making the rotating outside shell a slack fit with no friction to hold in place it could easily turn due to gravity pulling the leaves down.
To fix this i printed some tiny little clips that sit around the perimeter of the base putting friction between the base and rotating outside shell so it can't rotate without a deliberate twist :-) the clips are simply stuck in place with a tiny dab of superglue.
Here is a little video show the clips and how well they work compared to the same iris box in my link in the comment above. https://www.dropbox.com/s/zy4ymwdvfzu028u/20170716_013324.mp4?dl=0